Asynchronously call sys.exit() to avoid deadlock due to the JVM shutdown hooks

I finally managed to figure out why when ```suicide()``` was called by ```disconnected()``` that Marathon (i.e. the JVM) would not exit and would require a ```kill -9``` to get rid of the JVM process.

The issue was that ```suicide()``` called ```sys.exit()``` which initiates the JVM shutdown sequence. Part of the JVM shutdown sequence is to run shutdown hooks. This method will block until the shutdown hooks have ran and are finished. One of the shutdown hooks is to shutdown Marathon via ```mesosphere.chaos.App``` trait (which is used in ```Main.scala```). This stops all the services started with ```mesosphere.chaos.App.run()```. In ```Main.scala``` two services are run; ```HttpService``` and ```MarathonSchedulerService```. Thus, the shutdown hook calls ```MarathonSchedulerService.shutDown()``` which then calls ```MarathonSchedulerService.triggerShutdown()```. You will notice that ```triggerShutdown()``` tries to shutdown the driver (i.e. ```MesosSchedulerDriver```). But it can't stop the driver because the driver thread is currently blocked by ```sys.exit()``` in the ```suicide()``` method. And hence we have deadlock.

The simple solution is to put the ```sys.exit()``` call in its own thread so that it doesn't block the driver thread. And thus everything shuts down nicely :).