Add support for job priority to opcodes and job queue objects

This allows clients to submit opcodes with a priority. Except for being
tracked by the job queue, it is not yet used by any code.

Unittests for jqueue._QueuedOpCode and jqueue._QueuedJob are provided for
the first time.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>