python - fe: Use new pq/pqueue APIs and improve interrupt safety.

Log Message:
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Use new pq/pqueue APIs and improve interrupt safety.

While there is still much work to do around the read and write areas, this
change helps make KeyboardInterrupts "less" fatal(ugh).

Remove the "nowait" feature in favor of using aux(Interchange.insert) and the
new, yet-to-be-implemented step() method.

Modified Files:
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fe/src:
tracenull.py (r1.27 -> r1.28)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/fe/src/tracenull.py.diff?r1=1.27&r2=1.28)
fe/test:
tracenull.py (r1.14 -> r1.15)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/fe/test/tracenull.py.diff?r1=1.14&r2=1.15)