Job queue, matrix builder, concurrency control

[f-f]

WIP, should fix #696, fix #649

[f-f]

Main chunks of work to be done at this point:

• enqueuing/running matrix jobs, need to settle on a design for this. Current direction is "build plan is fixed at submission time, and we dynamically add jobs for dependent packages after a job is completed"
• enqueing/running package set jobs, need to figure out auth story for that
• return jobs from the API; need to spec it more:
  • do we keep backwards compatibility? Spago should keep working
  • what details do we return?
  • when returning a list we should allowing selecting queued/done/in progress jobs

[fsoikin]

But the first two bullets can be implemented as separate changes, right? They're not needed for strict parity.