SCTP: Introducing dcsctp, a new C++ SCTP implementation from WebRTC

Added meson build file to build dcsctp, which includes its minimal
dependencies of abseil-cpp and rtc_base module.

A thin C layer (sctpsocket) on top of dcsctp has been added to interop with the
library.

Removed usrsctp in favor of dcsctp.

dcsctp version:
https://webrtc.googlesource.com/src/+/61c5e86dca59b0ac8240444e7df5c31da27ee40f

sctpenc/sctpdec:
The sctpenc and sctpdec remains almost intact, only operations over the
sctp-association has been changed, as now they are not allowed to
ref/unref the association.
The elements should not have any direct impact on the associations lifetime.

sctp-factory:
Introduced a sctp-factory which manages the lifetime of the associations
and also manages the main loop and context in which all associations
opertions will live.
All the associations are handled in a single main loop now.

sctp-association:
The sctp-association has been restructured to use the new dcsctp socket
API.
The internal mutex of the sctp-association has been changed to be a
recursive mutex. In dcsctp, the association is the one that provides the
timers to the socket, which eventually can call back into the
association itself.

subprojects/gst-plugins-bad/ext/sctp/dcsctp/.clang-format

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subprojects/gst-plugins-bad/ext/sctp/dcsctp/absl/algorithm/algorithm.h

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+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: algorithm.h
+// -----------------------------------------------------------------------------
+//
+// This header file contains Google extensions to the standard <algorithm> C++
+// header.
+
+#ifndef ABSL_ALGORITHM_ALGORITHM_H_
+#define ABSL_ALGORITHM_ALGORITHM_H_
+
+#include <algorithm>
+#include <iterator>
+#include <type_traits>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace algorithm_internal {
+
+// Performs comparisons with operator==, similar to C++14's `std::equal_to<>`.
+struct EqualTo {
+  template <typename T, typename U>
+  bool operator()(const T& a, const U& b) const {
+    return a == b;
+  }
+};
+
+template <typename InputIter1, typename InputIter2, typename Pred>
+bool EqualImpl(InputIter1 first1, InputIter1 last1, InputIter2 first2,
+               InputIter2 last2, Pred pred, std::input_iterator_tag,
+               std::input_iterator_tag) {
+  while (true) {
+    if (first1 == last1) return first2 == last2;
+    if (first2 == last2) return false;
+    if (!pred(*first1, *first2)) return false;
+    ++first1;
+    ++first2;
+  }
+}
+
+template <typename InputIter1, typename InputIter2, typename Pred>
+bool EqualImpl(InputIter1 first1, InputIter1 last1, InputIter2 first2,
+               InputIter2 last2, Pred&& pred, std::random_access_iterator_tag,
+               std::random_access_iterator_tag) {
+  return (last1 - first1 == last2 - first2) &&
+         std::equal(first1, last1, first2, std::forward<Pred>(pred));
+}
+
+// When we are using our own internal predicate that just applies operator==, we
+// forward to the non-predicate form of std::equal. This enables an optimization
+// in libstdc++ that can result in std::memcmp being used for integer types.
+template <typename InputIter1, typename InputIter2>
+bool EqualImpl(InputIter1 first1, InputIter1 last1, InputIter2 first2,
+               InputIter2 last2, algorithm_internal::EqualTo /* unused */,
+               std::random_access_iterator_tag,
+               std::random_access_iterator_tag) {
+  return (last1 - first1 == last2 - first2) &&
+         std::equal(first1, last1, first2);
+}
+
+template <typename It>
+It RotateImpl(It first, It middle, It last, std::true_type) {
+  return std::rotate(first, middle, last);
+}
+
+template <typename It>
+It RotateImpl(It first, It middle, It last, std::false_type) {
+  std::rotate(first, middle, last);
+  return std::next(first, std::distance(middle, last));
+}
+
+}  // namespace algorithm_internal
+
+// equal()
+//
+// Compares the equality of two ranges specified by pairs of iterators, using
+// the given predicate, returning true iff for each corresponding iterator i1
+// and i2 in the first and second range respectively, pred(*i1, *i2) == true
+//
+// This comparison takes at most min(`last1` - `first1`, `last2` - `first2`)
+// invocations of the predicate. Additionally, if InputIter1 and InputIter2 are
+// both random-access iterators, and `last1` - `first1` != `last2` - `first2`,
+// then the predicate is never invoked and the function returns false.
+//
+// This is a C++11-compatible implementation of C++14 `std::equal`.  See
+// https://en.cppreference.com/w/cpp/algorithm/equal for more information.
+template <typename InputIter1, typename InputIter2, typename Pred>
+bool equal(InputIter1 first1, InputIter1 last1, InputIter2 first2,
+           InputIter2 last2, Pred&& pred) {
+  return algorithm_internal::EqualImpl(
+      first1, last1, first2, last2, std::forward<Pred>(pred),
+      typename std::iterator_traits<InputIter1>::iterator_category{},
+      typename std::iterator_traits<InputIter2>::iterator_category{});
+}
+
+// Overload of equal() that performs comparison of two ranges specified by pairs
+// of iterators using operator==.
+template <typename InputIter1, typename InputIter2>
+bool equal(InputIter1 first1, InputIter1 last1, InputIter2 first2,
+           InputIter2 last2) {
+  return absl::equal(first1, last1, first2, last2,
+                     algorithm_internal::EqualTo{});
+}
+
+// linear_search()
+//
+// Performs a linear search for `value` using the iterator `first` up to
+// but not including `last`, returning true if [`first`, `last`) contains an
+// element equal to `value`.
+//
+// A linear search is of O(n) complexity which is guaranteed to make at most
+// n = (`last` - `first`) comparisons. A linear search over short containers
+// may be faster than a binary search, even when the container is sorted.
+template <typename InputIterator, typename EqualityComparable>
+bool linear_search(InputIterator first, InputIterator last,
+                   const EqualityComparable& value) {
+  return std::find(first, last, value) != last;
+}
+
+// rotate()
+//
+// Performs a left rotation on a range of elements (`first`, `last`) such that
+// `middle` is now the first element. `rotate()` returns an iterator pointing to
+// the first element before rotation. This function is exactly the same as
+// `std::rotate`, but fixes a bug in gcc
+// <= 4.9 where `std::rotate` returns `void` instead of an iterator.
+//
+// The complexity of this algorithm is the same as that of `std::rotate`, but if
+// `ForwardIterator` is not a random-access iterator, then `absl::rotate`
+// performs an additional pass over the range to construct the return value.
+template <typename ForwardIterator>
+ForwardIterator rotate(ForwardIterator first, ForwardIterator middle,
+                       ForwardIterator last) {
+  return algorithm_internal::RotateImpl(
+      first, middle, last,
+      std::is_same<decltype(std::rotate(first, middle, last)),
+                   ForwardIterator>());
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_ALGORITHM_ALGORITHM_H_