[PATCH] Function to get size of asynchronous notification queue

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.