pgsql: Add new function pg_notification_queue_usage.

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a04bb65f70dafdf462e0478ad19e6de56df89bfc

Modified Files
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doc/src/sgml/func.sgml | 19 ++++++++--
doc/src/sgml/ref/notify.sgml | 5 +++
src/backend/commands/async.c | 50 ++++++++++++++++++++------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.h | 4 +++
src/include/commands/async.h | 1 +
src/test/isolation/expected/async-notify.out | 17 +++++++++
src/test/isolation/specs/async-notify.spec | 14 ++++++++
src/test/regress/expected/async.out | 8 +++++
src/test/regress/sql/async.sql | 4 +++
10 files changed, 110 insertions(+), 14 deletions(-)