[Request] Shut down Agent without exiting host JVM

[andrewazores]

Related #233

It should be possible to shut down the Agent cleanly without exiting the host JVM. This could be useful in one-off troubleshooting cases where the Agent functionality is only desired for a relatively short period of time, or in case the Agent causes some disruption to the host application JVM.

The first most obvious way to do this would be to use the existing SignalHandler to catch some user-configurable signal that the OpenJDK JVM does not already respond to by default. It could also be done by an authenticated WebServer request. In any case, the event should initiate an orderly shutdown of the Agent's webserver, harvester, and any other threads.