Add the "snapshot too old" feature

This feature is controlled by a new old_snapshot_threshold GUC.  A
value of -1 disables the feature, and that is the default.  The
value of 0 is just intended for testing.  Above that it is the
number of minutes a snapshot can reach before pruning and vacuum
are allowed to remove dead tuples which the snapshot would
otherwise protect.  The xmin associated with a transaction ID does
still protect dead tuples.  A connection which is using an "old"
snapshot does not get an error unless it accesses a page modified
recently enough that it might not be able to produce accurate
results.

This is similar to the Oracle feature, and we use the same SQLSTATE
and error message for compatibility.

src/include/utils/snapshot.h

@@ -14,6 +14,7 @@
 #define SNAPSHOT_H
 
 #include "access/htup.h"
+#include "access/xlogdefs.h"
 #include "lib/pairingheap.h"
 #include "storage/buf.h"
 
@@ -105,6 +106,9 @@ typedef struct SnapshotData
 	uint32		active_count;	/* refcount on ActiveSnapshot stack */
 	uint32		regd_count;		/* refcount on RegisteredSnapshots */
 	pairingheap_node ph_node;	/* link in the RegisteredSnapshots heap */
+
+	int64		whenTaken;		/* timestamp when snapshot was taken */
+	XLogRecPtr	lsn;			/* position in the WAL stream when taken */
 } SnapshotData;
 
 /*