jumpstart

[kazuho]

Builds on top of #452, and therefore this PR currently points to #452 as the base branch.

This PR implements Careful Resume WG draft-04 with following differences:

• Uses smoothed delivery rate (https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-careful-resume-04) and compares idle RTTs of the previous and new connection. That gives up better accuracy compared to using CWND and SRTT (a pair known to overshoot up to 2x at the end of slow start or undershoot by 1/2 immediately after a loss in congestion avoidance phase).
• When resuming, sends at the estimated bandwidth for full idle RTT. The draft states that endpoints should send at the estimated bandwidth for half the idle RTT. Justifications are that we have better accuracy (as stated above), and that there is no firm ground for the argument that half the idle RTT is better for fairness, considering that RTT of competing connections are going to be different.
• We count the number of bytes being acked among the packets sent in the jumpstart (i.e., unvalidated) phase, and if a loss is observed for any of the packets sent in that phase, CWND is adjusted to the number of bytes being acked. When the last packet sent in the unvalidated phase is acked (or deemed lost), CWND becomes equal to the pipe width.
• We do not have HyStart++ implemented.
• Separate knob (quicly_context_t::default_jumpstart_cwnd_bytes) is provided to set the CWND to be used when there is no previous information. This is useful for testing, et. al.
• We enter jumpstart (unvalidated phase) when we have more data to send than the pacer can send immediatly (i.e., 10 packets).

The PR adds following metrics to quicly_context_t:

• max_jumpstart_cwnd_bytes - cap for the CWND size used for jumpstart when previous information is available.
• default_jumpstart_cwnd_bytes jumpstart CWND to be used when no previous information is available; see above.

The PR adds following metrics to quicly_stats_t:

• jumpstart.prev_rate - delivery rate of the previous session
• jumpstart.prev_rtt - min RTT of the previous session
• jumpstart.new_rtt- min RTT of the new session
• jumpstart.cwnd - jumpstart CWND being adopted
• cc.exit_slow_start_at - contains milliseconds between creation of the connection and exit of slow start; alongside the existing parameter cc.cwnd_exiting_slow_start, this value would help us test the efficiency of jumpstart
• cc.cwnd_exiting_jumpstart - this value represents overestimation of the new RTT; if new RTT was overestimated, the value is going to be smaller than jumpstart.cwnd, as CWND is adjusted to the amount of bytes being acked previously, when exiting CWND (this is the moment when an the first ack is received for the packets sent in the unvalidated phase).