pgsql: Add a txid_status function.

Add a txid_status function.

If your connection to the database server is lost while a COMMIT is
in progress, it may be difficult to figure out whether the COMMIT was
successful or not. This function will tell you, provided that you
don't wait too long to ask. It may be useful in other situations,
too.

Craig Ringer, reviewed by Simon Riggs and by me

Discussion: http://postgr.es/m/CAMsr+YHQiWNEi0daCTboS40T+V5s_+dst3PYv_8v2wNVH+Xx4g@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/857ee8e391ff6654ef9dcc5dd8b658d7709d0a3c

Modified Files
--------------
doc/src/sgml/func.sgml | 27 ++++++
src/backend/utils/adt/txid.c | 132 ++++++++++++++++++++++++++++++
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.h | 2 +
src/test/recovery/t/011_crash_recovery.pl | 46 +++++++++++
src/test/regress/expected/txid.out | 68 +++++++++++++++
src/test/regress/sql/txid.sql | 38 +++++++++
7 files changed, 314 insertions(+), 1 deletion(-)