Fixes https://github.com/PostgREST/postgrest/issues/1512
Helps on environments where you can't send unix signals(Windows, managed
containers). Also provides better UX for schema reloads - NOTIFY
can be sent from pg clients(psql, pgadmin).
`NOTIFY pgrst` - with no payload - should be done to reload the schema cache.
Notifications with a payload will be ignored.
The channel can be enabled with `db-channel-enabled`(false by default)
and its name can be configured with `db-channel`.
The LISTEN thread uses a dedicated pg connection.
This connection is recovered if it fails.
A debounce of 1ms is done in case too many NOTIFYs arrive.
