message: bugfix once master rsync data to slave fail, it always fails
diff: @@ -773,9 +773,9 @@ void PikaServer::TryDBSync(const std::string& ip, int port, int32_t top) {
  void PikaServer::DBSync(const std::string& ip, int port) {
  // Only one DBSync task for every ip_port
  - std::string ip_port = slash::IpPortString(ip, port);
  {
  slash::MutexLock ldb(&db_sync_protector_);
  + std::string ip_port = slash::IpPortString(ip, port);
  if (db_sync_slaves_.find(ip_port) != db_sync_slaves_.end()) {
  return;
  }
  @@ -813,6 +813,9 @@ void PikaServer::DBSyncSendFile(const std::string& ip, int port) {
  LOG(INFO) << "Start Send files in " << bg_path << " to " << ip;
  ret = slash::GetChildren(bg_path, descendant);
  if (ret != 0) {
  + std::string ip_port = slash::IpPortString(ip, port);
  + slash::MutexLock ldb(&db_sync_protector_);
  + db_sync_slaves_.erase(ip_port);
  LOG(WARNING) << "Get child directory when try to do sync failed, error: " << strerror(ret);
  return;
  }
  @@ -864,8 +867,8 @@ void PikaServer::DBSyncSendFile(const std::string& ip, int port) {
  }
  // remove slave
  - std::string ip_port = slash::IpPortString(ip, port);
  {
  + std::string ip_port = slash::IpPortString(ip, port);
  slash::MutexLock ldb(&db_sync_protector_);
  db_sync_slaves_.erase(ip_port);
  }
  