Rework subtransaction commit protocol for hot standby.

This patch eliminates the marking of subtransactions as SUBCOMMITTED in pg_clog
during their commit; instead they remain in-progress until main transaction
commit.  At main transaction commit, the commit protocol is atomic-by-page
instead of one transaction at a time.  To avoid a race condition with some
subtransactions appearing committed before others in the case where they span
more than one pg_clog page, we conserve the logic that marks them subcommitted
before marking the parent committed.

Simon Riggs with minor help from me

src/backend/access/transam/twophase.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *		$PostgreSQL: pgsql/src/backend/access/transam/twophase.c,v 1.45 2008/08/11 11:05:10 heikki Exp $
+ *		$PostgreSQL: pgsql/src/backend/access/transam/twophase.c,v 1.46 2008/10/20 19:18:18 alvherre Exp $
  *
  * NOTES
  *		Each global transaction is associated with a global transaction
@@ -1745,9 +1745,7 @@ RecordTransactionCommitPrepared(TransactionId xid,
 	XLogFlush(recptr);
 
 	/* Mark the transaction committed in pg_clog */
-	TransactionIdCommit(xid);
-	/* to avoid race conditions, the parent must commit first */
-	TransactionIdCommitTree(nchildren, children);
+	TransactionIdCommitTree(xid, nchildren, children);
 
 	/* Checkpoint can proceed now */
 	MyProc->inCommit = false;
@@ -1822,8 +1820,7 @@ RecordTransactionAbortPrepared(TransactionId xid,
 	 * Mark the transaction aborted in clog.  This is not absolutely necessary
 	 * but we may as well do it while we are here.
 	 */
-	TransactionIdAbort(xid);
-	TransactionIdAbortTree(nchildren, children);
+	TransactionIdAbortTree(xid, nchildren, children);
 
 	END_CRIT_SECTION();
 }