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

src/test/regress/expected/hs_standby_allowed.out

@@ -208,6 +208,9 @@ LOCK hs1 IN ACCESS SHARE MODE;
 LOCK hs1 IN ROW SHARE MODE;
 LOCK hs1 IN ROW EXCLUSIVE MODE;
 COMMIT;
+-- UNLISTEN
+UNLISTEN a;
+UNLISTEN *;
 -- LOAD
 -- should work, easier if there is no test for that...
 -- ALLOWED COMMANDS

src/test/regress/expected/hs_standby_disallowed.out

@@ -118,10 +118,6 @@ listen a;
 ERROR:  cannot execute LISTEN during recovery
 notify a;
 ERROR:  cannot execute NOTIFY during recovery
-unlisten a;
-ERROR:  cannot execute UNLISTEN during recovery
-unlisten *;
-ERROR:  cannot execute UNLISTEN during recovery
 -- disallowed commands
 ANALYZE hs1;
 ERROR:  cannot execute ANALYZE during recovery

src/test/regress/sql/hs_standby_allowed.sql

@@ -110,6 +110,10 @@ LOCK hs1 IN ROW SHARE MODE;
 LOCK hs1 IN ROW EXCLUSIVE MODE;
 COMMIT;
 
+-- UNLISTEN
+UNLISTEN a;
+UNLISTEN *;
+
 -- LOAD
 -- should work, easier if there is no test for that...
 

src/test/regress/sql/hs_standby_disallowed.sql

@@ -88,8 +88,6 @@ COMMIT;
 -- Listen
 listen a;
 notify a;
-unlisten a;
-unlisten *;
 
 -- disallowed commands