Finish changes to stop an engine

Added a call to stop all react scripts, by passing all known routing
keys to the react_killer function. Then stop the threadpool executor.
Finally, Raise a known exception to stop the watchdog thread. This
will throw an ugly traceback, but will shutdown the engine
gracefully.

Also made minor changes to the example react.json, to change the log
format. Knowing the time a log was printed is useful.

Change-Id: Ibed06f79547312d188feb499f937eb5390d60c3e