Graceful shutdown - Should allow background thread to terminate

See nabble
http://camel.465427.n5.nabble.com/ShutdownTask-thread-does-not-complete-tp5756412.html

If you suspend/resume or stop/start routes etc then the graceful shutdown is in use to handle that. And it uses a background thread to do this work. But once that thread was created it would stay around forever as the core pool size is 1. But we should allow that thread to terminate.