jqueue/mcpu: Determine priority using callback

Instead of being given the priority for acquiring locks by means of a
parameter, mcpu will now call back. This is in preparation for
implementing a command to change a job's priority on the fly and allows
to change it while locks are being acquired (taking effect on the next
lock acquire).

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