Transaction safe Truncate

Ok.  Transaction safe truncate is simply transaction safe cluster
without the data copy and a slightly different set of permission checks.

I'd like to split cluster_rel() in cluster.c into 2 functions at line
174.  The permission checks, locking, etc will remain in cluster_rel(). 
The bottom half will be turned into a function called
rebuild_rel(tableOid Oid, indexOid Oid, dataCopy bool).

If dataCopy is set to false, then indexOid may be null -- this will
truncate the table.

Cluster will set dataCopy to true which will maintain current
expectations for cluster.

I'll also move TruncateRelation into cluster.c.

PreCommit_on_commit_actions() -> ONCOMMIT_DELETE_ROWS is the only 
location using heap_truncate(). It may be possible to change this and
remove heap_truncate() altogether.

-- 
Rod Taylor <rbt@rbt.ca>