Removed unused waitForKey() method from PlayRun

The method was copied to the PlayInteractionMode object in an earlier
commit. It should have been removed at that time, but must have been
forgotten.
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.
Merge pull request #1711 from richdougherty/run-thread-leak

Shut down Play threads in dev mode to prevent memory leaks