The bug fixed by 0874d4f3
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 546c13e1.

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