Use InvalidSnapshot, now SnapshotNow, as the default snapshot.

As far as I can determine, there's no code in the core distribution
that fails to explicitly set the snapshot of a scan or executor
state.  If there is any such code, this will probably cause it to
seg fault; friendlier suggestions were discussed on pgsql-hackers,
but there was no consensus that anything more than this was
needed.

This is another step towards the hoped-for complete removal of
SnapshotNow.

src/backend/access/index/genam.c

@@ -79,7 +79,7 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
 
 	scan->heapRelation = NULL;	/* may be set later */
 	scan->indexRelation = indexRelation;
-	scan->xs_snapshot = SnapshotNow;	/* may be set later */
+	scan->xs_snapshot = InvalidSnapshot;	/* caller must initialize this */
 	scan->numberOfKeys = nkeys;
 	scan->numberOfOrderBys = norderbys;
 

src/backend/executor/execUtils.c

@@ -105,7 +105,7 @@ CreateExecutorState(void)
 	 * Initialize all fields of the Executor State structure
 	 */
 	estate->es_direction = ForwardScanDirection;
-	estate->es_snapshot = SnapshotNow;
+	estate->es_snapshot = InvalidSnapshot;	/* caller must initialize this */
 	estate->es_crosscheck_snapshot = InvalidSnapshot;	/* no crosscheck */
 	estate->es_range_table = NIL;
 	estate->es_plannedstmt = NULL;