code: func (sh *SyncHandler) Fetch(context.Context, blob.Ref) (file io.ReadCloser, size uint32, err error) {
  	panic("unimplemented blobserver.Fetch called")
  }
docs: // TODO(bradfitz): implement these? what do they mean? possibilities:
  // a) proxy to sh.from
  // b) proxy to sh.to
  // c) merge intersection of sh.from, sh.to, and sh.queue: that is, a blob this pair
  //    currently or eventually will have. The only missing blob would be one that
  //    sh.from has, sh.to doesn't have, and isn't in the queue to be replicated.
  //
  // For now, don't implement them. Wait until we need them.