pgsql: Simplify and improve ProcessStandbyHSFeedbackMessage logic.

Simplify and improve ProcessStandbyHSFeedbackMessage logic.

There's no need to clamp the standby's xmin to be greater than
GetOldestXmin's result; if there were any such need this logic would be
hopelessly inadequate anyway, because it fails to account for
within-database versus cluster-wide values of GetOldestXmin.  So get rid of
that, and just rely on sanity-checking that the xmin is not wrapped around
relative to the nextXid counter.  Also, don't reset the walsender's xmin if
the current feedback xmin is indeed out of range; that just creates more
problems than we already had.  Lastly, don't bother to take the
ProcArrayLock; there's no need to do that to set xmin.

Also improve the comments about this in GetOldestXmin itself.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/b4a0223d008d7c2c9824d846e22b664b2f09cf6e

Modified Files
--------------
src/backend/replication/walsender.c |  105 ++++++++++++++++-------------------
src/backend/storage/ipc/procarray.c |   50 ++++++++++------
2 files changed, 79 insertions(+), 76 deletions(-)