Share the jqueue lock on job-local changes

We can share the jqueue lock when we do per-job updates. These only
conflict with updates/checks on the same job from another thread (eg.
CancelJob, ArchiveJob, which keep the lock unshared, since they are less
frequent).

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>