Stop prodding OBS backend when state is building
When OBS is told a worker is building it thinks it’s "building for
someone else" and marks in as away. See bs_repserver for more details.
Return cached worker info immediately, update in background
Workers already update their state when it changes, and this state
propagates through OBS proxies, so we can safely return it immediately
when requested by the warden, and schedule an update in the background.
This should mitigate timeouts when the warden prods OBS proxy server prods OBS proxy client prods worker, and while the reply climbs back through the interpipes, warden times out and kills the job.