Fix an undetected deadlock due to apply worker.

The apply worker needs to update the state of the subscription tables to
'READY' during the synchronization phase which requires locking the
corresponding subscription. The apply worker also waits for the
subscription tables to reach the 'SYNCDONE' state after holding the locks
on the subscription and the wait is done using WaitLatch. The 'SYNCDONE'
state is changed by tablesync workers again by locking the corresponding
subscription. Both the state updates use AccessShareLock mode to lock the
subscription, so they can't block each other. However, a backend can
simultaneously try to acquire a lock on the same subscription using
AccessExclusiveLock mode to alter the subscription. Now, the backend's
wait on a lock can sneak in between the apply worker and table sync worker
causing deadlock.

In other words, apply_worker waits for tablesync worker which waits for
backend, and backend waits for apply worker. This is not detected by the
deadlock detector because apply worker uses WaitLatch.

The fix is to release existing locks in apply worker before it starts to
wait for tablesync worker to change the state.

Reported-by: Tomas Vondra
Author: Shlok Kyal
Reviewed-by: Amit Kapila, Peter Smith
Backpatch-through: 12
Discussion: https://postgr.es/m/d291bb50-12c4-e8af-2af2-7bb9bb4d8e3e@enterprisedb.com

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