QUIC: optimized acknowledgement generation.

QUIC: optimized acknowledgement generation.

For application level packets, only every second packet is now acknowledged,
respecting max ack delay.

13.2.1 Sending ACK Frames

   In order to assist loss detection at the sender, an endpoint SHOULD
   generate and send an ACK frame without delay when it receives an ack-
   eliciting packet either:

   *  when the received packet has a packet number less than another
      ack-eliciting packet that has been received, or

   *  when the packet has a packet number larger than the highest-
      numbered ack-eliciting packet that has been received and there are
      missing packets between that packet and this packet.

13.2.2.  Acknowledgement Frequency

    A receiver SHOULD send an ACK frame after receiving at least two
    ack-eliciting packets.