= servlet: add termination awaiting to `Initializer::contextDestroyed`

Release memory by making sure all threads are stopped before app undeployment takes place.

More info at:
https://groups.google.com/forum/#!topic/spray-user/4wwQbHFXkxk
https://groups.google.com/forum/#!topic/akka-user/Lp2hn4FfhU8