Fix race conditions and missed wakeups in syncrep worker signaling.

When a sync worker is waiting for the associated apply worker to notice
that it's in SYNCWAIT state, wait_for_worker_state_change() would just
patiently wait for that to happen.  This generally required waiting for
the 1-second timeout in LogicalRepApplyLoop to elapse.  Kicking the worker
via its latch makes things significantly snappier.

While at it, fix race conditions that could potentially result in crashes:
we can *not* call logicalrep_worker_wakeup_ptr() once we've released the
LogicalRepWorkerLock, because worker->proc might've been reset to NULL
after we do that (indeed, there's no really solid reason to believe that
the LogicalRepWorker slot even belongs to the same worker anymore).
In logicalrep_worker_wakeup(), we can just move the wakeup inside the
lock scope.  In process_syncing_tables_for_apply(), a bit more code
rearrangement is needed.

Also improve some nearby comments.

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