Advance restart_lsn more eagerly in LogicalConfirmReceivedLocation

The existing logic computed an updated replicationSlotMinLSN from all
slots' restart_lsn only when catalog_xmin also advanced.  This is not a
problem in normal (non-repack) cases, because catalog_xmin changes
pretty frequently, so the recomputation is triggered frequently enough.
However, REPACK does not currently change its catalog snapshot, so that
doesn't work very well if no other replication slot is being used.

(After this commit, we still don't recycle WAL properly for REPACK,
because its background worker is not advancing its restart_lsn either;
that will be fixed in a separate commit.  However, this preexisting
problem in older code is logically separate from that one.)

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/TY4PR01MB17718B44164522D0798F8E898940A2@TY4PR01MB17718.jpnprd01.prod.outlook.com

src/backend/replication/logical/logical.c

@@ -1910,8 +1910,14 @@ LogicalConfirmReceivedLocation(XLogRecPtr lsn)
 			SpinLockRelease(&MyReplicationSlot->mutex);
 
 			ReplicationSlotsComputeRequiredXmin(false);
-			ReplicationSlotsComputeRequiredLSN();
 		}
+
+		/*
+		 * Now that the new restart_lsn is safely on disk, recompute the
+		 * global WAL retention requirement.
+		 */
+		if (updated_restart)
+			ReplicationSlotsComputeRequiredLSN();
 	}
 	else
 	{