Allow schema cache reloading with NOTIFY (#1542)

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.

CHANGELOG.md

@@ -8,6 +8,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 
  - #1525, Allow http status override through response.status guc - @steve-chavez
+ - #1512, Allow schema cache reloading with NOTIFY - @steve-chavez
 
 ### Fixed