Make DDL operations play nicely with Serializable Snapshot Isolation.

Truncating or dropping a table is treated like deletion of all tuples, and
check for conflicts accordingly. If a table is clustered or rewritten by
ALTER TABLE, all predicate locks on the heap are promoted to relation-level
locks, because the tuple or page ids of any existing tuples will change and
won't be valid after rewriting the table. Arguably ALTER TABLE should be
treated like a mass-UPDATE of every row, but if you e.g change the datatype
of a column, you could also argue that it's just a change to the physical
layout, not a logical change. Reindexing promotes all locks on the index to
relation-level lock on the heap.

Kevin Grittner, with a lot of cosmetic changes by me.

src/include/storage/predicate_internals.h

@@ -273,9 +273,7 @@ typedef struct PREDICATELOCKTARGETTAG
  * up the targets as the related tuples are pruned or vacuumed, we check the
  * xmin on access.	This should be far less costly.
  */
-typedef struct PREDICATELOCKTARGET PREDICATELOCKTARGET;
-
-struct PREDICATELOCKTARGET
+typedef struct PREDICATELOCKTARGET
 {
 	/* hash key */
 	PREDICATELOCKTARGETTAG tag; /* unique identifier of lockable object */
@@ -283,7 +281,7 @@ struct PREDICATELOCKTARGET
 	/* data */
 	SHM_QUEUE	predicateLocks; /* list of PREDICATELOCK objects assoc. with
 								 * predicate lock target */
-};
+} PREDICATELOCKTARGET;
 
 
 /*