How do cohorts get formed? How do tasks get assigned to workers?

[jlewi]

We need a mechanism for assigning tasks to workers.

• Coordinator shouldn't be able to assign tasks to specific workers; that would compromise privacy and go against the purpose of placements like tff.Clients.

• We need some mechanism to identify whether a given worker can claim a given task

  • This could be done with labels and selectors

• We need to form a cohort of a given size and make sure a given worker only gets a single task in that cohort

[jlewi]

Corresponding tech note. https://docs.google.com/document/d/1T8b8Ga_ORf283FeEsz1RshsxmIhJYkbzg2j2-_WS57w/edit#heading=h.tgf0yqghramm

The code is implemented but lacking an E2E test its hard to know whether this is working or not.

[jlewi]

E2E test is passing.

Follow on work in #20