Resolves #4315 and
#3800 This PR
automatically switches overlay/herder to using BucketListDB snapshots
instead of LedgerTxn.

This change introduces a single "read-only ledger state snapshot"
interface, which supports both SQL (via LedgerTxn) and BucketListDB
snapshots. A unified interface avoids invasive changes to the rest of
the codebase to support both snapshot types, and allows to have a
centralized validation flow for both overlay and apply time.

A notable change is the removal of nested LegderTxn in validation paths,
and using loadWithoutRecord instead in newer protocols. This creates a
stronger invariant that validation flow is only meant to be accessing
read-only version of the ledger, and prevents it from making
modifications by accident. Note that nested LedgerTxn is preserved for
older buggy versions of the protocol (making the code a bit uglier than
I would have preferred, I'm open to ideas on simplifying/refactoring old
protocol logic out of the critical path).

Part of #4318 - main
change is to stop passing around Application during validation and
application flows, and use AppConnector instead, which is forced to
either assert main thread or implement thread-safe methods

Resolves #3800 - note that
READ_ONLY_WITHOUT_SQL_TXN LedgerTxn mode should go away completely once
we switch to mandatory BucketListDB. It looks like we were using RO
LedgerTxn during apply scenarios, which I think is not legal? Either
way, all READ_ONLY_WITHOUT_SQL_TXN usages except for the legacy one in
LedgerSnapshot have been removed now.

Note that the second commit has a huge diff but is a no-op, as it's
basically a find and replace for `Application -> AppConnector`