feat: introduce pool acquisition timeout (fixes #2348)

The configuration option db-pool-acquisition-timeout
specifies the time in seconds to wait for the pool to
free up a connection slot. Otherwise, a 504 error is
returned. By default, there is no timeout.

src/PostgREST/AppState.hs

@@ -97,7 +97,10 @@ destroy = destroyPool
 
 initPool :: AppConfig -> IO SQL.Pool
 initPool AppConfig{..} =
-  SQL.acquire configDbPoolSize Nothing $ toUtf8 configDbUri
+  SQL.acquire configDbPoolSize timeoutMilliseconds $ toUtf8 configDbUri
+  where
+    timeoutMilliseconds = (* oneSecond) <$> configDbPoolAcquisitionTimeout
+    oneSecond = 1000000
 
 -- | Run an action with a database connection.
 usePool :: AppState -> SQL.Session a -> IO (Either SQL.UsageError a)