pgsql: Revert back-branch changes in behavior of age(xid).

Revert back-branch changes in behavior of age(xid).

Per discussion, it does not seem like a good idea to change the behavior of
age(xid) in a minor release, even though the old definition causes the
function to fail on hot standby slaves.  Therefore, revert commit
5829387381d2e4edf84652bb5a712f6185860670 and follow-on commits in the back
branches only.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/d9e1ea4de8c96c913bd979c1209258157a4605d9

Modified Files
--------------
src/backend/access/transam/xact.c |   24 ------------------------
src/backend/utils/adt/xid.c       |    4 ++--
src/include/access/xact.h         |    1 -
3 files changed, 2 insertions(+), 27 deletions(-)