any LISTEN command.  This is more important than it used to be because
DISCARD ALL invokes UNLISTEN.  Connection-pooled applications making heavy
use of DISCARD ALL were seeing significant contention for pg_listener,
as reported by Matteo Beccati.  It seems unlikely that clients using LISTEN
would use pooled connections, so this simple tweak seems sufficient,
especially since the pg_listener implementation is slated to go away soon
anyway.

Back-patch to 8.3, where DISCARD ALL was introduced.