pgsql: Allow UNLISTEN in hot-standby mode.

Allow UNLISTEN in hot-standby mode.

Since LISTEN is (still) disallowed, UNLISTEN must be a no-op in a
hot-standby session, and so there's no harm in allowing it.  This
change allows client code to not worry about whether it's connected
to a primary or standby server when performing session-state-reset
type activities.  (Note that DISCARD ALL, which includes UNLISTEN,
was already allowed, making it inconsistent to reject UNLISTEN.)

Per discussion, back-patch to all supported versions.

Shay Rojansky, reviewed by Mi Tar

Discussion: https://postgr.es/m/CADT4RqCf2gA_TJtPAjnGzkC3ZiexfBZiLmA-mV66e4UyuVv8bA@mail.gmail.com

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/741ee48900c5bbf970f23d32e49020408e417bdc

Modified Files
--------------
doc/src/sgml/high-availability.sgml                 | 11 ++++++++---
src/backend/tcop/utility.c                          |  2 +-
src/test/regress/expected/hs_standby_allowed.out    |  3 +++
src/test/regress/expected/hs_standby_disallowed.out |  4 ----
src/test/regress/sql/hs_standby_allowed.sql         |  4 ++++
src/test/regress/sql/hs_standby_disallowed.sql      |  2 --
6 files changed, 16 insertions(+), 10 deletions(-)