allow inspection of job manager thread state

[soxofaan]

Users can only start a job manager thread, and stop it, but they also might want to inspect the state: is it still running? and do something with that.

e.g. @mbuchhorn was playing with something like this:

while not manager._stop_thread:
    if not manager._stop_thread:
        visualize_something()
    time.sleep(15)

it should be possible to do that without using private _stop_thread