jqueue: Allow changing of job priority

This is due to a feature request. Sometimes one wants to change the
priority of a job after it has been submitted, e.g. after submitting an
important job only to later notice many other pending jobs which will be
processed first. Priority changes only take effect at the next lock
acquisition or when the job is re-scheduled.

The design is very similar to how jobs are cancelled.

Unit tests for “_QueuedJob.ChangePriority” are included.

Also rename “TestQueuedJob.test” to “TestQueuedJob.testError”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>