python - pq: Remove the Block interface and fix bugs in Passive.

Log Message:
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Remove the Block interface and fix bugs in Passive.

Block was inherently flawed as exceptions thrown by the second read invocation
would potentially cause unrecoverable state loss. That's bad.
(There are exceptions here, but it tends to require much work from the user)

Fix bugs in passive that would cause state-loss in low memory situations.

Modified Files:
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pq/src:
buffer.c (r1.5 -> r1.6)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/buffer.c.diff?r1=1.5&r2=1.6)
buffer.py (r1.4 -> r1.5)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/buffer.py.diff?r1=1.4&r2=1.5)

Added Files:
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pq/src:
pbuffer.py (r1.1)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/pbuffer.py?rev=1.1&content-type=text/x-cvsweb-markup)