Implement SKIP LOCKED for row-level locks

This clause changes the behavior of SELECT locking clauses in the
presence of locked rows: instead of causing a process to block waiting
for the locks held by other processes (or raise an error, with NOWAIT),
SKIP LOCKED makes the new reader skip over such rows.  While this is not
appropriate behavior for general purposes, there are some cases in which
it is useful, such as queue-like tables.

Catalog version bumped because this patch changes the representation of
stored rules.

Reviewed by Craig Ringer (based on a previous attempt at an
implementation by Simon Riggs, who also provided input on the syntax
used in the current patch), David Rowley, and Álvaro Herrera.

Author: Thomas Munro

src/include/access/heapam.h

@@ -19,6 +19,7 @@
 #include "nodes/primnodes.h"
 #include "storage/bufpage.h"
 #include "storage/lock.h"
+#include "utils/lockwaitpolicy.h"
 #include "utils/relcache.h"
 #include "utils/snapshot.h"
 
@@ -144,7 +145,7 @@ extern HTSU_Result heap_update(Relation relation, ItemPointer otid,
 			CommandId cid, Snapshot crosscheck, bool wait,
 			HeapUpdateFailureData *hufd, LockTupleMode *lockmode);
 extern HTSU_Result heap_lock_tuple(Relation relation, HeapTuple tuple,
-				CommandId cid, LockTupleMode mode, bool nowait,
+				CommandId cid, LockTupleMode mode, LockWaitPolicy wait_policy,
 				bool follow_update,
 				Buffer *buffer, HeapUpdateFailureData *hufd);
 extern void heap_inplace_update(Relation relation, HeapTuple tuple);