RPC to Get the List of Keys Associated with an MSA

[wilwade]

Caching services and others checking signatures off-chain want to know what keys are associated with a given MSA Id.

There is no state that maps MSA Ids to Account Ids. Only Account Ids to MSA Ids.

We only need to be able to retrieve the list of Account Ids for a given MSA id off-chain.

Acceptance Criteria

• Custom RPC get_msa_keys
  • Param: msaId
  • Response: Array

Arch Notes

• Create an off-chain database store that keeps track of MSAId -> Account Id

  • Idea: Off-chain worker (events)
  • Idea: Off-chain worker (state (accountid -> MSAId) for initial + events for upkeep)
  • Idea: Other? (Parity didn't have any)
  • Options around offchain (regular, offchain indexing)

• Blocked by spike: [Spike] on offchain storage and query #474

• Related: [State Optimization] Refactor MsaKeysOf to store a reference counter #384