refactor: move socket creation and management to App module

Right now listening sockets initialization, management and usage is split between App, AppState and Admin modules: they are created in AppState.init and remembered in AppState but used only in App and Admin.

It has several negative consequences:
- sockets are initialized even if not needed (eg. command line invocations like dump-config or dump-schema)
- it is impossible to start listening on a socket after initial schema cache load because it requires AppState

This change decouples listen socket management from AppState. Sockets are created only when needed (ie. not in command line tools invocation) and passed to admin application and to Warp by the App module.

src/PostgREST/Admin.hs

@@ -19,22 +19,23 @@ import PostgREST.Observation (Observation (..))
 
 import qualified PostgREST.AppState as AppState
 
-import Protolude
+import qualified Network.Socket as NS
+import           Protolude
 
-runAdmin :: AppState -> Warp.Settings -> IO ()
-runAdmin appState settings = do
-  whenJust (AppState.getSocketAdmin appState) $ \adminSocket -> do
+runAdmin :: AppState -> Maybe NS.Socket -> NS.Socket -> Warp.Settings -> IO ()
+runAdmin appState maybeAdminSocket socketREST settings = do
+  whenJust maybeAdminSocket $ \adminSocket -> do
     address <- resolveSocketToAddress adminSocket
     observer $ AdminStartObs address
     void . forkIO $ Warp.runSettingsSocket settings adminSocket adminApp
   where
-    adminApp = admin appState
+    adminApp = admin appState socketREST
     observer = AppState.getObserver appState
 
 -- | PostgREST admin application
-admin :: AppState.AppState -> Wai.Application
-admin appState req respond  = do
-  isMainAppReachable  <- isRight <$> reachMainApp (AppState.getSocketREST appState)
+admin :: AppState.AppState -> NS.Socket -> Wai.Application
+admin appState socketREST req respond  = do
+  isMainAppReachable  <- isRight <$> reachMainApp socketREST
   isLoaded <- AppState.isLoaded appState
   isPending <- AppState.isPending appState