upgrade to fork of hasql-pool 0.7.2 (fixes #2401)

This version of hasql-pool is a simplified rewrite that doesn't use
the resource-pool package. The major API changes are that idle
connections are no longer timed out (and the corresponding setting
is gone), and that `release` makes the pool unusable, where it used
to remain usable and only flushed idle connections.

We depend on a PostgREST fork of 0.7.2 that gives us reliable
flushing, compare https://github.com/PostgREST/hasql-pool/pull/1

- hasql-pool 0.7 removes timing out of idle connections, so
  this change removes the db-pool-timeout option.
  Given that we were typically running with very high
  timeout settings, I don't anticipate the lack of timeout
  to introduce new issues, though we might want to consider
  introducing some retry-logic down the line when we
  encounter connection failures.
- See https://github.com/PostgREST/postgrest/issues/2422 for a
  discussion on depending on a forked dependency. Besides adding
  the dependency to the nix overlay, we're also adding it to
  stack.yaml and a new cabal.project to allow stack/cabal users
  to build the project.

src/PostgREST/AppState.hs

@@ -74,12 +74,12 @@ data AppState = AppState
 
 init :: AppConfig -> IO AppState
 init conf = do
-  newPool <- initPool conf
-  initWithPool newPool conf
+  pool <- initPool conf
+  initWithPool pool conf
 
 initWithPool :: SQL.Pool -> AppConfig -> IO AppState
-initWithPool newPool conf =
-  AppState newPool
+initWithPool pool conf =
+  AppState pool
     <$> newIORef minimumPgVersion -- assume we're in a supported version when starting, this will be corrected on a later step
     <*> newIORef Nothing
     <*> newIORef mempty
@@ -93,23 +93,24 @@ initWithPool newPool conf =
     <*> newIORef 0
 
 destroy :: AppState -> IO ()
-destroy AppState{..} = SQL.release statePool
+destroy = destroyPool
 
 initPool :: AppConfig -> IO SQL.Pool
 initPool AppConfig{..} =
-  SQL.acquire (configDbPoolSize, configDbPoolTimeout, toUtf8 configDbUri)
+  SQL.acquire configDbPoolSize $ toUtf8 configDbUri
 
+-- | Run an action with a database connection.
 usePool :: AppState -> SQL.Session a -> IO (Either SQL.UsageError a)
 usePool AppState{..} = SQL.use statePool
 
 -- | Flush the connection pool so that any future use of the pool will
 -- use connections freshly established after this call.
---
--- FIXME: #2401 Connections that are in-use during the call to flushPool
--- will currently be returned to the pool and reused afterwards, in
--- conflict with the intention.
 flushPool :: AppState -> IO ()
-flushPool AppState{..} = SQL.release statePool
+flushPool AppState{..} = SQL.flush statePool
+
+-- | Destroy the pool on shutdown.
+destroyPool :: AppState -> IO ()
+destroyPool AppState{..} = SQL.release statePool
 
 getPgVersion :: AppState -> IO PgVersion
 getPgVersion = readIORef . statePgVersion