Replace the KnownAssignedXids hash table with a sorted-array data structure,

and be more tense about the locking requirements for it, to improve performance
in Hot Standby mode.  In passing fix a few bugs and improve a number of
comments in the existing HS code.

Simon Riggs, with some editorialization by Tom

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 1.60 2010/04/13 14:17:46 heikki Exp $
+ *		$PostgreSQL: pgsql/src/backend/access/transam/twophase.c,v 1.61 2010/04/28 00:09:05 tgl Exp $
  *
  * NOTES
  *		Each global transaction is associated with a global transaction
@@ -1200,6 +1200,9 @@ StandbyTransactionIdIsPrepared(TransactionId xid)
 
 	Assert(TransactionIdIsValid(xid));
 
+	if (max_prepared_xacts <= 0)
+		return false;					/* nothing to do */
+
 	/* Read and validate file */
 	buf = ReadTwoPhaseFile(xid, false);
 	if (buf == NULL)