====== When to send an acknowledgment? ======

From quic-transport-24 draft, section 13.2.
>   Endpoints acknowledge all packets they receive and process.  However,
>   only ack-eliciting packets cause an ACK frame to be sent within the
>   maximum ack delay.  Packets that are not ack-eliciting are only
>   acknowledged when an ACK frame is sent for other reasons.
>   
>   When sending a packet for any reason, an endpoint should attempt to
>   bundle an ACK frame if one has not been sent recently.

and further from section 13.2.1.
>   Every packet SHOULD be acknowledged at least once, and ack-eliciting
>   packets MUST be acknowledged at least once within the maximum ack
>   delay.
>   
>   An ACK frame SHOULD be generated for at least every second ack-
>   eliciting packet.
>   
>   An endpoint that is only sending ACK frames will not receive
>   acknowledgments from its peer unless *those acknowledgements* are
>   included in packets with ack-eliciting frames.  An endpoint SHOULD
>   bundle ACK frames with other frames when there are new ack-eliciting
>   packets to acknowledge.  When only non-ack-eliciting packets need to
>   be acknowledged, an endpoint MAY wait until an ack-eliciting packet
>   has been received to bundle an ACK frame with outgoing frames.

This leads to the following implementation.

**on ack-eliciting packet received** -> start ack delay timer and notice that we could send an ack.
* when sending a packet -> include this ack and cancel ack delay timer.
* on second ack-eliciting packet received -> send ack immediately and cancel ack delay timer.
* on ack delay timeout -> send ack immediately.

**on non-ack-eliciting packet** -> ? - Let the parameter quic.bundleAckForNonAckElicitingPackets decide.

**on non-ack-eliciting packet** while bundleAckForNonAckElicitingPackets=true -> notice that we could send an ack, but do not start ack delay timer. 

* When sending a packet, include this ack.

**on non-ack-eliciting packet** while bundleAckForNonAckElicitingPackets=false -> do nothing