implement lazy servo, only move for significant delta_az values

[fergusL]

There really needs to be a tolerance on dome azimuth so that sufficiently small delta_az values do not trigger a move. A 'lazy servo' like this is common practice with dome controllers because it's often difficult to make small moves accurately. Without a tolerance the dome can end up hunting back and forth, endlessly chasing impossible precision.

Originally posted by @AnthonyHorton in https://github.com/_render_node/MDE3OlB1bGxSZXF1ZXN0UmV2aWV3MjkyNzg2ODM3/pull_request_reviews/more_threads

[AnthonyHorton]

It's possible that TheSkyX will already be doing this for us, in which case we don't want it being done at both ends.