Add new function pg_notification_queue_usage.

Robert Haas <rhaas@postgresql.org>

Commit: a04bb65f70dafdf462e0478ad19e6de56df89bfc
Author: Robert Haas <rhaas@postgresql.org>
Date: 2015-07-17T13:12:03Z
Releases: 9.6.0
Add new function pg_notification_queue_usage.

This tells you what fraction of NOTIFY's queue is currently filled.

Brendan Jurd, reviewed by Merlin Moncure and Gurjeet Singh.  A few
further tweaks by me.

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Documentation touched