improve ManagedProcess cleanup of resources

[pnwamk]

The ManagedProcess class currently only relies on __del__ to clean up its underlying process, which can be unpredictable. Having such code as a failsafe may be reasonable, but we should add more explicit methods for cleaning things up in a predictable manner.

Thanks @kquick for pointing out.

[pnwamk]

Summary of initial suggestion from a quick peruse from Kevin:

For subclasses of ManagedProcess shutdown might need to be subclass-specific.

Perhaps add @abstractmethod to ServerProcess for the shutdown and go from there,
being aware Python's weak add-on typing won't lead you to all the places that might 
need to call that method.

[pnwamk]

N.B., this has the potential to affect the non-HTTP based connections. The HTTP connections rely almost entirely on the requests.post method for all comms handling.