Add PRR 6937bis heuristic and remove prr_conservative sysctl

[rscheff]

Summary

Proportional Rate Reduction (RFC6937) is in the process
of becoming a standard. The difference to the experimental
RFC is a heuristic, which automatically chooses between
CRB and SSRB schema. Only when snd_una advances (a partial
ACK), SSRB may be used. Also, that ACK must not have
any indication of ongoing loss - using the addition of
new holes into the scoreboard as proxy for such an
event.

Now based off D29441.

MFC after: 2 weeks

[rscheff]

The code above is my current understanding of the 6927bis draft. Note that this will make PRR loss recovery slightly less aggressive especially under heavy loss.

Also, this change probably doesn't change much in the pathological instance discueed in the draft, as the base stack has no capability at the moment, to retransmit lost retransmissions, and will instead run into an RTO and slow-start thereafter.

[rscheff]

• rebase to HEAD

[rscheff]

rebase against D29441

[rscheff]

• use PRR-SSRB and PRR-CRB per -bis heuristic

[rscheff]

• update comment to include ACK split attack

[rscheff]

no magic numbers in sack-changed.

[rscheff]

• rebase without D29441
• make magic values human-readable