python - be: New constants and IST interfaces Add subtransaction

Log Message:
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New constants and IST interfaces

Add subtransaction interfaces with *very* basic support for protecting
over-Release/Rollback from FATAL'ing. Postgres.Begin, Postgres.Release, and
Postgres.Rollback.

Change Get-TransactionId/CommandID/UserId to use Postgres.Object's.
Forget about NotPyPgTransaction.

Modified Files:
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be/src:
module.c (r1.8 -> r1.9)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/module.c.diff?r1=1.8&r2=1.9)
constants.c (r1.4 -> r1.5)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/constants.c.diff?r1=1.4&r2=1.5)