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/overlays/haskell-packages/default.nix

@@ -12,6 +12,19 @@ let
             ver = "0.3.0";
             sha256 = "0iwh4wsjhb7pms88lw1afhdal9f86nrrkkvv65f9wxbd1b159n72";
           } { };
+      # To get the sha256
+      # nix-prefetch-url --unpack https://hackage.haskell.org/package/hasql-notifications-0.1.0.0/hasql-notifications-0.1.0.0.tar.gz
+      hasql-notifications =
+        self.haskell.lib.overrideCabal
+          (
+            prev.callHackageDirect
+              {
+                pkg = "hasql-notifications";
+                ver = "0.1.0.0";
+                sha256 = "1z17gsqvvzzi0yipc3qy3jz8vzpww4vsc4vaj2kbzr2mfliq6fx3";
+              } { }
+          )
+          (old: { doCheck = false; });
     } // extraOverrides final prev;
 in
 {