* BUG SYMPTOM: when deleting a very large channel, the call to
  `phoneNumberRegistrar.destroy` fails due to a mysterious `Operation
  timeout`
* LIKELY CAUSE: awaiting the resolution of the call to
  `notifyMembers` (which takes over an hour for a large channel)
  likely causes the transaction initiated at the top of `destroy` to
  timeout
* FIX: launch notifications as "fire and forget" process in the
  background and add some more verbose logging to help debug if error persists