Replace the pg_listener-based LISTEN/NOTIFY mechanism with an in-memory queue.

Replace the pg_listener-based LISTEN/NOTIFY mechanism with an in-memory queue.
In addition, add support for a "payload" string to be passed along with
each notify event.

This implementation should be significantly more efficient than the old one,
and is also more compatible with Hot Standby usage.  There is not yet any
facility for HS slaves to receive notifications generated on the master,
although such a thing is possible in future.

Joachim Wieland, reviewed by Jeff Davis; also hacked on by me.