* extract a `phoneNumberRegistrar.redeem`, which may be called from
  dispatcher as soon as a redemption message is received. it:
  * deletes recycle request for redeemed channel
  * immediately notifies channel admins and instance maintainers that
    channel has been redeemed (no more waiting 24 hours! :))
* refactor `recycleRequestRepository.evaluateRecycleRequests`
  * rename to `recycleRequest.getMaturerecycleRequests`
  * only retrieve and return mature recycle requests
  * don't worry about filtering out redeemed channel phone numbers
    anymore, since this will be done atomically and directly by calls
    to `#redeem` above
* simplify `phoneNumberRegistrar.processRecycleRequests`:
  * don't worry about redeemed nubmers
  * just get the mature requests, recycle their channels, and notify
    the admins that this happened