pgsql: PL/Python explicit subtransactions

PL/Python explicit subtransactions

Adds a context manager, obtainable by plpy.subtransaction(), to run a
group of statements in a subtransaction.

Jan Urbański, reviewed by Steve Singer, additional scribbling by me

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/22690719ea5a89ccbcd04deb58c83d8d5f138df8

Modified Files
--------------
doc/src/sgml/plpython.sgml                         |  132 +++++++-
src/pl/plpython/Makefile                           |    1 +
src/pl/plpython/expected/README                    |    2 +
.../plpython/expected/plpython_subtransaction.out  |  378 ++++++++++++++++++++
.../expected/plpython_subtransaction_0.out         |  366 +++++++++++++++++++
src/pl/plpython/expected/plpython_test.out         |    6 +-
src/pl/plpython/plpython.c                         |  283 +++++++++++++++-
src/pl/plpython/sql/plpython_subtransaction.sql    |  244 +++++++++++++
8 files changed, 1404 insertions(+), 8 deletions(-)