pgsql: Fix incomplete backpatch of pg_multixact truncation changes to <

Fix incomplete backpatch of pg_multixact truncation changes to <= 9.2

The backpatch of a95335b544d9c8377e9dc7a399d8e9a155895f82 to 9.2, 9.1
and 9.0 was incomplete, missing changes to xlog.c, primarily the call
to TrimMultiXact(). Testing presumably didn't show a problem without
these changes because TrimMultiXact() performs defense-in-depth work,
which is not strictly necessary.

It also missed moving StartupMultiXact() which would have been
problematic if a restartpoing happened in exactly the wrong moment,
causing a transient error.

Andres Freund

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/3d974e303da29ff97822212ab8722d6e366c5119

Modified Files
--------------
src/backend/access/transam/xlog.c | 13 ++++++++++---
src/include/access/multixact.h | 1 +
2 files changed, 11 insertions(+), 3 deletions(-)