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.

test/SpecHelper.hs

@@ -69,6 +69,10 @@ _baseCfg =  -- Connection Settings
             Nothing
             -- No user configured Unix Socket file mode (defaults to 660)
             (Right 432)
+            -- db-channel
+            "pgrst"
+            -- db-channel-enabled
+            False
             -- Jwt settings
             (Just $ encodeUtf8 "reallyreallyreallyreallyverysafe") False Nothing
             -- Connection Modifiers
@@ -88,7 +92,7 @@ _baseCfg =  -- Connection Settings
             []
 
 testCfg :: Text -> AppConfig
-testCfg testDbConn = _baseCfg { configDatabase = testDbConn }
+testCfg testDbConn = _baseCfg { configDbUri = testDbConn }
 
 testCfgNoJWT :: Text -> AppConfig
 testCfgNoJWT testDbConn = (testCfg testDbConn) { configJwtSecret = Nothing }