Object (Domain) locking

Sorry, sending this wile packing to leave for a week... It should be ok.

- Modifies LOCKTAG to include a 'classId'. Relation receive a classId of
RelOid_pg_class, and transaction locks XactLockTableId. RelId is renamed
to objId.

- LockObject() and UnlockObject() functions created, and their use
sprinkled throughout the code to do descent locking for domains and
types. They accept lock modes AccessShare and AccessExclusive, as we
only really need a 'read' and 'write' lock at the moment. Most locking
cases are held until the end of the transaction.

This fixes the cases Tom mentioned earlier in regards to locking with
Domains. If the patch is good, I'll work on cleaning up issues with
other database objects that have this problem (most of them).

--
Rod Taylor <rbt(at)rbt(dot)ca>

PGP Key: http://www.rbt.ca/rbtpub.asc