Beaker should support sending messages on a message bus for each (un-finished) job when the status changes. Ideally we would send a message for every sub-component down to the recipe-task level. We could also consider sending a notification for every new recipe-task-result added, but it might make sense to handle that separately.

This would let consumers more efficiently wait for their job status to change, rather than polling in a loop.

bkr job-watch is an example of such a consumer, and it might make sense to port it over to consume these messages as part of this bug too. (It will need to support a polling loop too, for Beaker sites where no message bus is available.)