Design protocol & related structures for onboarding and offboarding validators

[piotr-roslaniec]

• Key share refreshing ceremony refactor #175 adds an intermediate solution to offboarding validators, Dkg.offboard_validator(address)
• To ship the cohort refreshing and recovery schemes, we need to reliably track the state of Coordinator in the local Dkg instances
• Consider stateless Dkg implementation, Remove the remainder of statfulness from the DKG instances #108
• Related: Track domain points (aka "x coordinates") mapping to validators #160

[piotr-roslaniec]

This issue has been fixed by the changes introduced in #175 and #182 - The DKG instances now expect Coordinator to track the validator set and will not track it internally. The validator set is accessed through methods exposed to server-side API. We expect that the server-side implementation will keep track of the validator set and ensure its correctness.