Add support for LISTEN/NOTIFY.

Using db.Query("LISTEN relname"), returning an
infinite Rows object. This is not optimal as a
Rows.Next() call cannot be interrupted, but should
fit the API well.
Make LISTEN/NOTIFY use channels.

This is now a completely separate API, not using database.sql. There
is code duplication from conn.go, but this is a first PoC.

Supports reconnection and notifications on reconnect.
Fix refs to RelName and Extra fields