[PATCH] Prevent repeated deadlock-check signals in standby buffer pin waits

In ResolveRecoveryConflictWithBufferPin(), when deadlock_timeout fires,
the function sends RECOVERY_CONFLICT_BUFFERPIN_DEADLOCK and returns.
The caller (LockBufferForCleanup) loops back, sets up another
deadlock_timeout,
and the signal gets sent again every interval.

The lock-conflict path had the same problem and was fixed in 8900b5a9d59a
by adding a second ProcWaitForSignal() after the deadlock-check signal.
The buffer-pin path was left with an XXX comment asking "should we fix
this?".

The attached patch applies the same fix: after sending the deadlock-check
signal, reset got_standby_deadlock_timeout and call ProcWaitForSignal()
so the startup process waits for UnpinBuffer() rather than looping
and re-signaling.

Patch attached.