Feature Request: better support for LISTEN/NOTIFY?

Currently, you have to execute a dummy query like `SELECT 1` every time you want to get a hold of new notifications from the server. Can anything similar to aiopg be implemented in asyncpg?

https://aiopg.readthedocs.io/en/stable/examples.html#usage-of-listen-notify-commands
https://www.psycopg.org/docs/connection.html#connection.poll

Start by reviewing asyncpg's existing LISTEN/NOTIFY handling, then compare the aiopg usage example and psycopg connection.poll documentation linked in the issue. Done means clients can receive new server notifications without issuing a dummy SELECT 1 query each time.

Written by the indexing model from the issue text.