Remove dependency on HeapTuple from predicate locking functions.

The following changes make the predicate locking functions more
generic and suitable for use by future access methods:

- PredicateLockTuple() is renamed to PredicateLockTID().  It takes
  ItemPointer and inserting transaction ID instead of HeapTuple.

- CheckForSerializableConflictIn() takes blocknum instead of buffer.

- CheckForSerializableConflictOut() no longer takes HeapTuple or buffer.

Author: Ashwin Agrawal
Reviewed-by: Andres Freund, Kuntal Ghosh, Thomas Munro
Discussion: https://postgr.es/m/CALfoeiv0k3hkEb3Oqk%3DziWqtyk2Jys1UOK5hwRBNeANT_yX%2Bng%40mail.gmail.com

src/backend/access/hash/hashinsert.c

@@ -88,7 +88,7 @@ restart_insert:
 										  &usedmetap);
 	Assert(usedmetap != NULL);
 
-	CheckForSerializableConflictIn(rel, NULL, buf);
+	CheckForSerializableConflictIn(rel, NULL, BufferGetBlockNumber(buf));
 
 	/* remember the primary bucket buffer to release the pin on it at end. */
 	bucket_buf = buf;