Post-commit review fixes for slot synchronization.

Allow pg_sync_replication_slots() to error out during promotion of standby.
This makes the behavior of the SQL function consistent with the slot sync
worker. We also ensured that pg_sync_replication_slots() cannot be
executed if sync_replication_slots is enabled and the slotsync worker is
already running to perform the synchronization of slots. Previously, it
would have succeeded in cases when the worker is idle and failed when it
is performing sync which could confuse users.

This patch fixes another issue in the slot sync worker where
SignalHandlerForShutdownRequest() needs to be registered *before* setting
SlotSyncCtx->pid, otherwise, the slotsync worker could miss handling
SIGINT sent by the startup process(ShutDownSlotSync) if it is sent before
worker could register SignalHandlerForShutdownRequest(). To be consistent,
all signal handlers' registration is moved to a prior location before we
set the worker's pid.

Ensure that we clean up synced temp slots at the end of
pg_sync_replication_slots() to avoid such slots being left over after
promotion.

Ensure that ShutDownSlotSync() captures SlotSyncCtx->pid under spinlock to
avoid accessing invalid value as it can be reset by concurrent slot sync
exit due to an error.

Author: Shveta Malik
Reviewed-by: Hou Zhijie, Bertrand Drouvot, Amit Kapila, Masahiko Sawada
Discussion: https://postgr.es/m/CAJpy0uBefXUS_TSz=oxmYKHdg-fhxUT0qfjASW3nmqnzVC3p6A@mail.gmail.com

Author: Amit Kapila

doc/src/sgml/func.sgml

@@ -29349,6 +29349,10 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
         standby server. Temporary synced slots, if any, cannot be used for
         logical decoding and must be dropped after promotion. See
         <xref linkend="logicaldecoding-replication-slots-synchronization"/> for details.
+        Note that this function cannot be executed if
+        <link linkend="guc-sync-replication-slots"><varname>
+        sync_replication_slots</varname></link> is enabled and the slotsync
+        worker is already running to perform the synchronization of slots.
        </para>
 
        <caution>