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/patches/static-haskell-nix-hasql-notifications-openssl-linking-fix.patch

@@ -0,0 +1,28 @@
+From 49ecb703d9d0bfd38eb69ba5cb63a8262bd03f96 Mon Sep 17 00:00:00 2001
+From: steve-chavez <stevechavezast@gmail.com>
+Date: Thu, 11 Jun 2020 13:18:07 -0500
+Subject: [PATCH] Add hasql-notifications openssl linking fix
+
+---
+ survey/default.nix | 5 +++++
+ 1 file changed, 5 insertions(+)
+
+diff --git a/survey/default.nix b/survey/default.nix
+index 828beaa..9c2d5f6 100644
+--- a/survey/default.nix
++++ b/survey/default.nix
+@@ -1054,6 +1054,11 @@ let
+                   super.squeal-postgresql
+                   [ final.openssl ]
+                   "--libs openssl";
++              hasql-notifications =
++                addStaticLinkerFlagsWithPkgconfig
++                  super.hasql-notifications
++                  [ final.openssl ]
++                  "--libs openssl";
+
+               xml-to-json =
+                 addStaticLinkerFlagsWithPkgconfig
+--
+2.19.3
+