jqueue: Change model from per-job to per-opcode processing

In order to support priorities, the processing of jobs needs to be
changed. Instead of processing jobs as a whole, the code is changed to
process one opcode at a time and then return to the queue. See the
Ganeti 2.3 design document for details.

This patch does not yet use priorities for acquiring locks.

The enclosed unittests increase the test coverage of jqueue.py from
about 34% to 58%. Please note that they also test some parts not added
by this patch, but testing them became only possible with some
infrastructure added by this patch. For the first time, many
implications and assumptions for the job queue are codified in these
unittests.

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