pgsql-server: Reduce pg_listener lock taken by NOTIFY et al from

Log Message:
-----------
Reduce pg_listener lock taken by NOTIFY et al from AccessExclusiveLock
to ExclusiveLock. This still serializes the operations of this module,
but doesn't conflict with concurrent ANALYZE operations. Per trouble
report from Philip Warner a few weeks ago.

Tags:
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REL7_4_STABLE

Modified Files:
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pgsql-server/src/backend/commands:
async.c (r1.102 -> r1.102.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/async.c.diff?r1=1.102&r2=1.102.2.1)