dnsdist: Always store the OpenSSLTLSIOCtx in the connection

[rgacogne]

Short description

Instead of storing the frontend context for server-side connection and the SSL_CTX* for client-side.

The first commit in this PR fixes a race condition where reloading the certificate and key via reloadAllCertificates(), for example, could cause the ALPN-related data to be destroyed while it was still used by an existing TLS connection. The time window to trigger the issue is quite narrow because ALPN data is only accessed during the initial handshake, but we have got one report of it happening in the wild, and TSAN confirms the race. This is not a security issue since it cannot be triggered without administrative access to dnsdist.
The remaining commits are making this code more clean and robust, and should probably not be backported.
I would recommend reviewing this PR commit per commit!

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 10882923159

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 62 of 66 (93.94%) changed or added relevant lines in 3 files are covered.
• 55 unchanged lines in 17 files lost coverage.
• Overall coverage increased (+0.002%) to 64.681%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/tcpiohandler.cc	49	53	92.45%

Files with Coverage Reduction	New Missed Lines	%
pdns/libssl.cc	1	59.01%
ext/lmdb-safe/lmdb-typed.hh	1	73.6%
pdns/packethandler.cc	1	72.51%
modules/gpgsqlbackend/gpgsqlbackend.cc	1	88.62%
modules/lmdbbackend/lmdbbackend.cc	2	73.54%
pdns/ws-auth.cc	2	80.8%
pdns/recursordist/test-syncres_cc2.cc	3	88.91%
pdns/misc.hh	3	87.62%
pdns/tsigverifier.cc	3	77.22%
pdns/fstrm_logger.cc	3	44.08%

Totals
Change from base Build 10881391740:	0.002%
Covered Lines:	124775
Relevant Lines:	162216

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💛 - Coveralls