Rework phased middleware, fix several bugs

Bugs fixed:

 - express helpers like `req.get` are now available in middleware
   handlers registered via `app.middleware`

 - `req.url` does not include the mountpath prefix now, this is
   consistent with the behaviour of `app.use`

The implementation of phased middleware was completely rewritten.
 - We no longer use Phase and PhaseList objects from loopback-phase.
 - Handler functions are registered via the `Layer` mechanism used by
   express router.
 - The app keeps the layers sorted according to phases.