refactor: move pool release on shutdown out of signal handler

This changes behaviour somewhat in that:
- We now consistently release the pool on shutdown, even on non-Unix
  platforms, and including for CmdDumpConfig.
- We release the pool *after* interrupting `App.run`, which will
  rather cause more than fewer connection to be closed properly.
  (Previously any in-use connections would not have been caught by
  `releasePool`, though *maybe* the `UserInterrupt` handling in
  the web handler ends up closing the connections properly already
  anyway).

(The main aim of the change is to make it clearer when and why the
pool is released.)

src/PostgREST/AppState.hs

@@ -2,6 +2,7 @@
 
 module PostgREST.AppState
   ( AppState
+  , destroy
   , getConfig
   , getDbStructure
   , getIsListenerOn
@@ -89,6 +90,9 @@ initWithPool newPool conf =
     <*> myThreadId
     <*> newIORef 0
 
+destroy :: AppState -> IO ()
+destroy = releasePool
+
 initPool :: AppConfig -> IO SQL.Pool
 initPool AppConfig{..} =
   SQL.acquire (configDbPoolSize, configDbPoolTimeout, toUtf8 configDbUri)