design-2.2: job queue lock analysis/remediation

This builds up on the "Master core scalability design doc" detailing the
critical situations in the job queue and proposing how to fix them. The
bulleted point list at the beginning is changed to subparagraph, as the
job queue part is quite longer and more detailed, then the remediation
section is updated removing the generic "we'll fix it somehow" paragraph
to propose a real solution.

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