Optimized scoreboard for SACK

DragonFly now has a optimized scoreboard for SACK, thanks to Sepherosa Ziehau.  What’s that mean?  SACK is a way to make sure only the needed parts of a TCP transmission get retransmitted, when multiple packets are lost.  The scoreboard is where the packets needing retransmission are tracked.  So, the result of these improvements is better performance in packet-lossy situations.

(Please correct me if your understanding is better than mine; my explanation is based on stumbling around the Internet for a few minutes of reading.)