handleExistConnections(socket) {
    const self = this;
    self.socketIndex++;
    const key = `socketIndex_${self.socketIndex}`;
    self.socketPool[key] = socket;

    // if the socket is closed already, removed it from pool
    socket.on('close', () => {
      delete self.socketPool[key];
    });
  }
manage all created socket
for each new socket, we put them to a map;
if the socket is closed itself, we remove it from the map
when the `close` method is called, we'll close the sockes before the server closed

@param {Socket} the http socket that is creating
@returns undefined
@memberOf ProxyCore