Optimize commit_siblings in two ways to improve group commit.

First, avoid scanning the whole ProcArray once we know there
are at least commit_siblings active; second, skip the check
altogether if commit_siblings = 0.

Greg Smith

src/backend/access/transam/xact.c

@@ -1052,7 +1052,7 @@ RecordTransactionCommit(void)
 		 * fewer than CommitSiblings other backends with active transactions.
 		 */
 		if (CommitDelay > 0 && enableFsync &&
-			CountActiveBackends() >= CommitSiblings)
+			MinimumActiveBackends(CommitSiblings))
 			pg_usleep(CommitDelay);
 
 		XLogFlush(XactLastRecEnd);