Shut down Play threads in dev mode to prevent memory leaks

* Introduce ClosableLazy class. Like Scala's lazy, but allows
  closing of an initializaed value too.
* Made Execution.internalContext and Invoker.system into
  ClosableLazy values and close them on Server.stop().
* Also made AkkaPlugin's system a ClosableLazy. It was already
  being closed, but it used its own ad-hoc closing system
  based on Scala's lazy.