optimize ReadHolder in MetaClient::getStorageLeaderFromCache

[HarrisChu]

StatusOr<HostAddr> MetaClient::getStorageLeaderFromCache(GraphSpaceID spaceId, PartitionID partId) {
  if (!ready_) {
    return Status::Error("Not ready!");
  }

  {
    folly::RWSpinLock::ReadHolder holder(leadersLock_);
    auto iter = leadersInfo_.leaderMap_.find({spaceId, partId});
    if (iter != leadersInfo_.leaderMap_.end()) {
      return iter->second;
    }
  }