python - pq: Fix potential deadlock.

Log Message:
-----------
Fix potential deadlock. Transaction.hooks was not protected by a lock. I think
this caused odd deadlocks from time to time by sneaking in a hook to the list
after it had been processed for finishing, which, of course, would be deadly to
Transaction.wait().

Modified Files:
--------------
    pq/src:
        transam.py (r1.3 -> r1.4)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/transam.py.diff?r1=1.3&r2=1.4)