Improve snapshot manager by keeping explicit track of snapshots.

There are two ways to track a snapshot: there's the "registered" list, which
is used for arbitrary long-lived snapshots; and there's the "active stack",
which is used for the snapshot that is considered "active" at any time.
This also allows users of snapshots to stop worrying about snapshot memory
allocation and freeing, and about using PG_TRY blocks around ActiveSnapshot
assignment.  This is all done automatically now.

As a consequence, this allows us to reset MyProc->xmin when there are no
more snapshots registered in the current backend, reducing the impact that
long-running transactions have on VACUUM.

src/include/utils/snapshot.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/snapshot.h,v 1.2 2008/03/26 21:10:39 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/utils/snapshot.h,v 1.3 2008/05/12 20:02:02 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -57,6 +57,9 @@ typedef struct SnapshotData
 	 * out any that are >= xmax
 	 */
 	CommandId	curcid;			/* in my xact, CID < curcid are visible */
+	uint32		active_count;	/* refcount on ActiveSnapshot stack */
+	uint32		regd_count;		/* refcount on RegisteredSnapshotList */
+	bool		copied;			/* false if it's a static snapshot */
 } SnapshotData;
 
 /*