promotion related handling in pg_sync_replication_slots()

Hello hackers,

Currently, promotion related handling is missing in the slot sync SQL
function pg_sync_replication_slots().   Here is the background on how
it is done in slot sync worker:
During promotion, the startup process in order to shut down the
slot-sync worker, sets the 'stopSignaled' flag, sends the shut-down
signal, and waits for slot sync worker to exit. Meanwhile if the
postmaster has not noticed the promotion yet, it may end up restarting
slot sync worker. In such a case, the worker exits if 'stopSignaled'
is set.

Since there is a chance that the user (or any of his scripts/tools)
may execute SQL function pg_sync_replication_slots() in parallel to
promotion, such handling is needed in this SQL function as well, The
attached patch attempts to implement the same. Changes are:

1) If pg_sync_replication_slots()  is already running when the
promotion is triggered, ShutDownSlotSync() checks the
'SlotSyncCtx->syncing' flag as well and waits for it to become false
i.e. waits till parallel running SQL function is finished.

2) If  pg_sync_replication_slots() is invoked when promotion is
already in progress, pg_sync_replication_slots() respects the
'stopSignaled' flag set by the startup process and becomes a no-op.

thanks
Shveta