Add a way to run a job upon reception of a notification

While I'm not sure if this would still fit in the scope of this nice extension, I'd like to propose extending the existing infrastructure to allow scheduling of jobs on LISTEN channels that would run upon reception of an notification raised by an NOTIFY command. That way (and in combination with something like the [tcn contrib module](https://www.postgresql.org/docs/current/static/tcn.html)) one could design asynchronous triggers or similar constructs.