Change extension of spilled ReorderBufferChange data to ".tmp".

The previous extension, ".snap", was chosen for historical reasons and
became confusing.

Discussion: https://postgr.es/m/CAMp0ubd_P8vBGx8=MfDXQJZxHA5D_Zarw5cCkDxJ_63+pWRJ9w@mail.gmail.com

src/backend/replication/logical/reorderbuffer.c

@@ -2765,7 +2765,7 @@ ReorderBufferCleanupSerializedTXNs(const char *slotname)
 			if (unlink(path) != 0)
 				ereport(ERROR,
 						(errcode_for_file_access(),
-						 errmsg("could not remove file \"%s\" during removal of pg_replslot/%s/*.xid: %m",
+						 errmsg("could not remove file \"%s\" during removal of pg_replslot/%s/xid*: %m",
 								path, slotname)));
 		}
 	}
@@ -2785,7 +2785,7 @@ ReorderBufferSerializedPath(char *path, ReplicationSlot *slot, TransactionId xid
 
 	XLogSegNoOffsetToRecPtr(segno, 0, wal_segment_size, recptr);
 
-	snprintf(path, MAXPGPATH, "pg_replslot/%s/xid-%u-lsn-%X-%X.snap",
+	snprintf(path, MAXPGPATH, "pg_replslot/%s/xid-%u-lsn-%X-%X.tmp",
 			 NameStr(MyReplicationSlot->data.name),
 			 xid,
 			 (uint32) (recptr >> 32), (uint32) recptr);