Scheduler should log all job creation, execution and cancellation

If a schedule never executes, it is almost impossible to debug why, because the scheduler does not emit any log messages. It should log - probably at debug level - when a job gets triggered (enters the job queue) and when it gets executed or cancelled (exits the job queue). It should also log (probably at trace level) any time a job in the job queue changes its state (for example, a constraint is fulfilled).