Fix leak with SMgrRelations in startup process

The startup process does not process shared invalidation messages, only
sending them, and never calls AtEOXact_SMgr() which clean up any
unpinned SMgrRelations.  Hence, it is never able to free SMgrRelations
on a periodic basis, bloating its hashtable over time.

Like the checkpointer and the bgwriter, this commit takes a conservative
approach by freeing periodically SMgrRelations when replaying a
checkpoint record, either online or shutdown, so as the startup process
has a way to perform a periodic cleanup.

Issue caused by 21d9c3ee4e, so backpatch down to v17.

Author: Jingtang Zhang <mrdrivingduck@gmail.com>
Reviewed-by: Yuhang Qiu <iamqyh@gmail.com>
Discussion: https://postgr.es/m/28C687D4-F335-417E-B06C-6612A0BD5A10@gmail.com
Backpatch-through: 17

src/backend/access/transam/xlog.c

@@ -8385,6 +8385,14 @@ xlog_redo(XLogReaderState *record)
 							checkPoint.ThisTimeLineID, replayTLI)));
 
 		RecoveryRestartPoint(&checkPoint, record);
+
+		/*
+		 * After replaying a checkpoint record, free all smgr objects.
+		 * Otherwise we would never do so for dropped relations, as the
+		 * startup does not process shared invalidation messages or call
+		 * AtEOXact_SMgr().
+		 */
+		smgrdestroyall();
 	}
 	else if (info == XLOG_CHECKPOINT_ONLINE)
 	{
@@ -8438,6 +8446,14 @@ xlog_redo(XLogReaderState *record)
 							checkPoint.ThisTimeLineID, replayTLI)));
 
 		RecoveryRestartPoint(&checkPoint, record);
+
+		/*
+		 * After replaying a checkpoint record, free all smgr objects.
+		 * Otherwise we would never do so for dropped relations, as the
+		 * startup does not process shared invalidation messages or call
+		 * AtEOXact_SMgr().
+		 */
+		smgrdestroyall();
 	}
 	else if (info == XLOG_OVERWRITE_CONTRECORD)
 	{