[PATCH] Function to get size of asynchronous notification queue

Hello hackers,

I present a patch to add a new built-in function
pg_notify_queue_saturation().

The purpose of the function is to allow users to monitor the health of
their notification queue. In certain cases, a client connection listening
for notifications might get stuck inside a transaction, and this would
cause the queue to keep filling up, until finally it reaches capacity and
further attempts to NOTIFY error out.

The current documentation under LISTEN explains this possible gotcha, but
doesn't really suggest a useful way to address it, except to mention that
warnings will show up in the log once you get to 50% saturation of the
queue. Unless you happen to be eyeballing the logs when it happens, that's
not a huge help. The choice of 50% as a threshold is also very much
arbitrary, and by the time you hit 50% the problem has likely been going on
for quite a while. If you want your nagios (or whatever) to say, alert you
when the queue goes over 5% or 1%, your options are limited and awkward.

The patch has almost no new code. It makes use of the existing logic for
the 50% warning. I simply refactored that logic into a separate function
asyncQueueSaturation, and then added pg_notify_queue_saturation to make
that available in SQL.

I am not convinced that pg_notify_queue_saturation is the best possible
name for this function, and am very much open to other suggestions.

The patch includes documentation, a regression test and an isolation test.

Cheers,
BJ