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/static-haskell-package.nix

@@ -18,6 +18,7 @@ let
       static-haskell-nix
       [
         patches.static-haskell-nix-postgrest-openssl-linking-fix
+        patches.static-haskell-nix-hasql-notifications-openssl-linking-fix
       ];
 
   patchedNixpkgs =