pgsql: Rework handling of subtransactions in 2PC recovery

Rework handling of subtransactions in 2PC recovery

The bug fixed by 0874d4f3e183757ba15a4b3f3bf563e0393dd9c2
caused us to question and rework the handling of
subtransactions in 2PC during and at end of recovery.
Patch adds checks and tests to ensure no further bugs.

This effectively removes the temporary measure put in place
by 546c13e11b29a5408b9d6a6e3cca301380b47f7f.

Author: Simon Riggs
Reviewed-by: Tom Lane, Michael Paquier
Discussion: http://postgr.es/m/CANP8+j+vvXmruL_i2buvdhMeVv5TQu0Hm2+C5N+kdVwHJuor8w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/49e928154978da2a5976628588fc545b726ad84a

Modified Files
--------------
src/backend/access/transam/subtrans.c | 32 +++++++++++------
src/backend/access/transam/twophase.c | 67 +++++++++++++++--------------------
src/backend/access/transam/xact.c | 2 +-
src/backend/access/transam/xlog.c | 4 +--
src/backend/storage/ipc/procarray.c | 2 +-
src/include/access/subtrans.h | 2 +-
src/include/access/twophase.h | 2 +-
7 files changed, 56 insertions(+), 55 deletions(-)