_flush_session read the unsynced messages, posted them, and only then set
_synced. in a short-lived run the async writer draining save()'s queue and
the exit-time flush_all() both saw the same batch unsynced, so every turn
of a one-shot run landed in honcho twice (#92458).
each HonchoSession now carries an RLock that _flush_session holds around
the whole select, send, mark sequence. the second flusher enters after the
first marked the batch and sends nothing; different sessions still flush
in parallel. the lock lives on the session object instead of a manager
dict keyed by session id (#86094, #92787): it needs no eviction, and two
flushers of one message list can never hold different locks. tests
adapted from #86094 and #92787.
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