Clean stop for widget server

[mustafasoylu]

Currently, the only way to stop the bokeh server is by doing sending sigterm (ctrl+c). There should be a function that uses the clean stop function.

I believe it can be easily implemented by adjusting the run function and add a stop function in widget server.

[apirogov]

The problem is not with the bokeh stop function, but with the IOLoop. Running the ioloop.start method blocks the thread. For a clean shutdown thus multithreading is needed, so that the process can somehow receive a stop (or restart) command, e.g. from a flask endpoint.

This is of course also doable, but its a bit more than just calling start and stop. But surely would be nice to have e.g. WidgetServer.run(block=False) or something like that (by default it would use the blocking way to run it)