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.