pgsql: Shut down transaction tracking at startup process exit.

Shut down transaction tracking at startup process exit.

Maxim Orlov reported that the shutdown of standby server could result in
the following assertion failure. The cause of this issue was that,
when the shutdown caused the startup process to exit, recovery-time
transaction tracking was not shut down even if it's already initialized,
and some locks the tracked transactions were holding could not be released.
At this situation, if other process was invoked and the PGPROC entry that
the startup process used was assigned to it, it found such unreleased locks
and caused the assertion failure, during the initialization of it.

    TRAP: FailedAssertion("SHMQueueEmpty(&(MyProc->myProcLocks[i]))"

This commit fixes this issue by making the startup process shut down
transaction tracking and release all locks, at the exit of it.

Back-patch to all supported branches.

Reported-by: Maxim Orlov
Author: Fujii Masao
Reviewed-by: Maxim Orlov
Discussion: https://postgr.es/m/ad4ce692cc1d89a093b471ab1d969b0b@postgrespro.ru

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dd5b7e486a4f90b54db62e57b84b81003c0d032d

Modified Files
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src/backend/postmaster/startup.c  | 19 +++++++++++++++++++
src/backend/storage/ipc/standby.c | 15 +++++++++++++++
2 files changed, 34 insertions(+)