python - pq: Add the 'Queue' class for managing protocol transactions.

Log Message:
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Add the 'Queue' class for managing protocol transactions.

This was originally in fe/tracenull.py, but it has been moved here as users of
'pq' are more than likely going to need something like this. The move here
demands some generalizations, so it's not an exact copy, but most of the
state-conscience code is the same. Notably, none of the thread safeties have been
carried over. This intentional omission was done in order to save cycles for
users that have no need for such safeties. Users in need of such a feature can
either subclass the Queue class or wrap instances with a protective object.

Modified Files:
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pq/src:
transam.py (r1.10 -> r1.11)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/transam.py.diff?r1=1.10&r2=1.11)