Add handler for LISTEN/NOTIFY notifications

[jordanlewis]

lib/pq currently has a dedicated set of functionality for subscribing to
LISTEN/NOTIFY messages, but you have to use it on a separate connection.

This commit adds a simple way to register a handler for notifications
that is on an ordinary database connection. This is useful if a user
wants to LISTEN to a channel on the same connection as their ordinary
database connection.

[jordanlewis]

@mjibson PTAL. This is for adding messages to cockroach sql when a connection receives a LISTEN/NOTIFY message. See cockroachdb/cockroach#48308.

[jordanlewis]

Thanks!