[493] WIP implement contact id resolution in identity/session stores

- testssss!
- there is currently a race in the first inbound message that we think
  can be resolved either by:
  - (1) making the process prekey helper suspend/async/await (so
    `processMessage` doesn't happen until it finishes)
  - (2) doing some refactoring of relationship btw/ ContactStore &
    AccountProtocol store so that ContactStore can know about the
    session lock (and by taking it, prevent session Store from trying to
    call `resolveContactId` before we've crated a contact)