wsOnDrainHandler(socket) {
    socket.cork(() => {
      const buffer = this.backpressureBuffer.get(socket);

      while (
        buffer.length > 0 &&
        socket.getBufferedAmount() < WS_PER_SOCKET_BACKPRESSURE_BUFFER_SIZE
      ) {
        const payload = buffer.shift();
        socket.send(payload);
      }
    });
  }
Absorb as much of the backpressure buffer as possible
@param  {uWS.WebSocket} socket