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.

nix/docker/postgrest.conf

@@ -12,6 +12,9 @@ db-pool = "$(PGRST_DB_POOL)"
 db-pool-timeout = "$(PGRST_DB_POOL_TIMEOUT)"
 db-extra-search-path = "$(PGRST_DB_EXTRA_SEARCH_PATH)"
 
+db-channel = "$(PGRST_DB_CHANNEL)"
+db-channel-enabled = "$(PGRST_DB_CHANNEL_ENABLED)"
+
 server-host = "$(PGRST_SERVER_HOST)"
 server-port = "$(PGRST_SERVER_PORT)"