Fix memory leak and inefficiency in CREATE DATABASE ... STRATEGY WAL_LOG

RelationCopyStorageUsingBuffer() did not free the strategies used to access
the source / target relation. They memory was released at the end of the
transaction, but when using a template database with a lot of relations, the
temporary leak can become big prohibitively big.

RelationCopyStorageUsingBuffer() acquired the buffer for the target relation
with RBM_NORMAL, therefore requiring a read of a block guaranteed to be
zero. Use RBM_ZERO_AND_LOCK instead.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/20230321070113.o2vqqxogjykwgfrr@awork3.anarazel.de
Backpatch: 15-, where STRATEGY WAL_LOG was introduced

Author: anarazel

src/backend/storage/buffer/bufmgr.c

@@ -3756,11 +3756,9 @@ RelationCopyStorageUsingBuffer(RelFileNode srcnode,
 		LockBuffer(srcBuf, BUFFER_LOCK_SHARE);
 		srcPage = BufferGetPage(srcBuf);
 
-		/* Use P_NEW to extend the destination relation. */
 		dstBuf = ReadBufferWithoutRelcache(dstnode, forkNum, blkno,
-										   RBM_NORMAL, bstrategy_dst,
+										   RBM_ZERO_AND_LOCK, bstrategy_dst,
 										   permanent);
-		LockBuffer(dstBuf, BUFFER_LOCK_EXCLUSIVE);
 		dstPage = BufferGetPage(dstBuf);
 
 		START_CRIT_SECTION();
@@ -3778,6 +3776,9 @@ RelationCopyStorageUsingBuffer(RelFileNode srcnode,
 		UnlockReleaseBuffer(dstBuf);
 		UnlockReleaseBuffer(srcBuf);
 	}
+
+	FreeAccessStrategy(bstrategy_src);
+	FreeAccessStrategy(bstrategy_dst);
 }
 
 /* ---------------------------------------------------------------------