Listen/Notify payload and interfaces

This one is for the maintainers of the various postgresql interfaces:

With the new listen/notify implementation we can send a payload along
with the notification. This has been in the protocol already since
years and there is no change needed for libpq. However we need to
adapt the various interfaces to allow a payload to be sent and
received.

A notification is sent via SQL:

http://developer.postgresql.org/pgdocs/postgres/sql-notify.html

and received via the libpq call PQnotifies:

http://developer.postgresql.org/pgdocs/postgres/libpq-notify.html

Joachim