upgrade to hasql-1.6, hasql-pool-0.8.0.2

- hasql-pool-0.8
  * 'release' now flushes the pool, and no longer destroys it;
    'PoolIsReleasedUsageError' is gone compared to our fork
  * now supports an acquisition timeout, which we don't use
    yet
  * lower bound on 0.8.0.2 to fix a Windows build issue
- hasql-1.6
  introduces a position parameter to ServerError, which we
  ignore

src/PostgREST/AppState.hs

@@ -97,7 +97,7 @@ destroy = destroyPool
 
 initPool :: AppConfig -> IO SQL.Pool
 initPool AppConfig{..} =
-  SQL.acquire configDbPoolSize $ toUtf8 configDbUri
+  SQL.acquire configDbPoolSize Nothing $ toUtf8 configDbUri
 
 -- | Run an action with a database connection.
 usePool :: AppState -> SQL.Session a -> IO (Either SQL.UsageError a)
@@ -106,7 +106,7 @@ 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.
 flushPool :: AppState -> IO ()
-flushPool AppState{..} = SQL.flush statePool
+flushPool AppState{..} = SQL.release statePool
 
 -- | Destroy the pool on shutdown.
 destroyPool :: AppState -> IO ()