status: do not create thread to run callbacks if timeout is None

[mguijarr]

Here is a merge request that mitigates concern expressed here: https://github.com/bluesky/ophyd/pull/837#discussion_r409007458 (also linked with issue #844).

I have the impression, a simple optimization is to not create a thread if it is not needed (in particular when timeout is set to None, which seems to be most of the time).

Otherwise some background about the problem Status objects want to solve with the callbacks thread would be insightful.

[tacaswell]

Thank you for this, this seems sensible to me and looks correct to me.

Can you add a test in test_status.py to cover the case with no timeout but a settle time?

[mguijarr]

@tacaswell Done

[tacaswell]

Thank you @mguijarr !