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.

src/PostgREST/Config.hs

@@ -67,7 +67,7 @@ import Protolude.Conv    (toS)
 
 -- | Config file settings for the server
 data AppConfig = AppConfig {
-    configDatabase          :: Text
+    configDbUri             :: Text
   , configAnonRole          :: Text
   , configOpenAPIProxyUri   :: Maybe Text
   , configSchemas           :: NonEmpty Text
@@ -75,6 +75,8 @@ data AppConfig = AppConfig {
   , configPort              :: Int
   , configSocket            :: Maybe FilePath
   , configSocketMode        :: Either Text FileMode
+  , configDbChannel         :: Text
+  , configDbChannelEnabled  :: Bool
 
   , configJwtSecret         :: Maybe B.ByteString
   , configJwtSecretIsBase64 :: Bool
@@ -163,6 +165,8 @@ readOptions = do
         <*> (fromMaybe 3000 <$> optInt "server-port")
         <*> (fmap unpack <$> optString "server-unix-socket")
         <*> parseSocketFileMode "server-unix-socket-mode"
+        <*> (fromMaybe "pgrst" <$> optString "db-channel")
+        <*> ((Just True ==) <$> optBool "db-channel-enabled")
         <*> (fmap encodeUtf8 <$> optString "jwt-secret")
         <*> ((Just True ==) <$> optBool "secret-is-base64")
         <*> parseJwtAudience "jwt-aud"
@@ -276,6 +280,11 @@ readOptions = do
           |## when none is provided, 660 is applied by default
           |# server-unix-socket-mode = "660"
           |
+          |## Notification channel for reloading the schema cache
+          |# db-channel = "pgrst"
+          |## Enable or disable the notification channel
+          |# db-channel-enabled = false
+          |
           |## base url for swagger output
           |# openapi-server-proxy-uri = ""
           |