code: func (sl *ServerLookup) ServerAddr(id raft.ServerID) (raft.ServerAddress, error) {
  	sl.lock.RLock()
  	defer sl.lock.RUnlock()
  	svr, ok := sl.idToServer[id]
  	if !ok {
  		return "", fmt.Errorf("Could not find address for server id %v", id)
  	}
  	return raft.ServerAddress(svr.Addr.String()), nil
  }
docs: // Implements the ServerAddressProvider interface