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 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.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>