Improve the performance of relation deletes during recovery.

When multiple relations are deleted at the same transaction,
the files of those relations are deleted by one call to smgrdounlinkall(),
which leads to scan whole shared_buffers only one time. OTOH,
previously, during recovery, smgrdounlink() (not smgrdounlinkall()) was
called for each file to delete, which led to scan shared_buffers
multiple times. Obviously this could cause to increase the WAL replay
time very much especially when shared_buffers was huge.

To alleviate this situation, this commit changes the recovery so that
it also calls smgrdounlinkall() only one time to delete multiple
relation files.

This is just fix for oversight of commit 279628a0a7, not new feature.
So, per discussion on pgsql-hackers, we concluded to backpatch this
to all supported versions.

Author: Fujii Masao
Reviewed-by: Michael Paquier, Andres Freund, Thomas Munro, Kyotaro Horiguchi, Takayuki Tsunakawa
Discussion: https://postgr.es/m/CAHGQGwHVQkdfDqtvGVkty+19cQakAydXn1etGND3X0PHbZ3+6w@mail.gmail.com

src/backend/access/transam/twophase.c

@@ -1456,7 +1456,6 @@ FinishPreparedTransaction(const char *gid, bool isCommit)
 	RelFileNode *delrels;
 	int			ndelrels;
 	SharedInvalidationMessage *invalmsgs;
-	int			i;
 
 	/*
 	 * Validate the GID, and lock the GXACT to ensure that two backends do not
@@ -1549,13 +1548,9 @@ FinishPreparedTransaction(const char *gid, bool isCommit)
 		delrels = abortrels;
 		ndelrels = hdr->nabortrels;
 	}
-	for (i = 0; i < ndelrels; i++)
-	{
-		SMgrRelation srel = smgropen(delrels[i], InvalidBackendId);
 
-		smgrdounlink(srel, false);
-		smgrclose(srel);
-	}
+	/* Make sure files supposed to be dropped are dropped */
+	DropRelationFiles(delrels, ndelrels, false);
 
 	/*
 	 * Handle cache invalidation messages.