python - pq: Improve interrupt safety.

Log Message:
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Improve interrupt safety.

Transitions are made interrupt safe by making a successful transition come down
to a single point; iff that point is passed, the transition is complete,
otherwise the step must be performed again. It is up to the user to identify
when a message needs to be processed again(Perhaps some assistance here may
be in order).

Additionally, change the Transaction's API to used regular named methods. After
some -m timeit sessions, named methods appear to be fetched and executed faster
than __call__/__lshift__/etc. (I imagine Python optimizes the common case.)

Modified Files:
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pq/src:
client3.py (r1.22 -> r1.23)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/client3.py.diff?r1=1.22&r2=1.23)
element3.py (r1.14 -> r1.15)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/element3.py.diff?r1=1.14&r2=1.15)
transam.py (r1.8 -> r1.9)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/transam.py.diff?r1=1.8&r2=1.9)
pq/test:
client3.py (r1.9 -> r1.10)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/test/client3.py.diff?r1=1.9&r2=1.10)