pgsql: Reset master xmin when hot_standby_feedback disabled.

Reset master xmin when hot_standby_feedback disabled.
If walsender has xmin of standby then ensure we
reset the value to 0 when we change from hot_standby_feedback=on
to hot_standby_feedback=off.

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/2dde11a632d3fe309b5af5480d01a0a3028f7f64

Modified Files
--------------
doc/src/sgml/protocol.sgml            |    5 +++-
src/backend/replication/walreceiver.c |   45 +++++++++++++++++++++++++--------
src/backend/replication/walsender.c   |    5 +++-
3 files changed, 42 insertions(+), 13 deletions(-)