Every call to sender.NextSeg does not need to iterate from the
front of the writeList as in a given recovery episode we can cache
the last nextSeg returned. There cannot be a lower sequenced segment
that matches the next call to NextSeg as otherwise we would have
returned that instead in the previous call.
This fixes the issue of excessive CPU usage w/ large send buffers
where we spend a lot of time iterating from the front of the list on
every NextSeg invocation.
Further the following other bugs were also fixed:
* Iteration of segments never sent in NextSeg() when looking for segments for
retransmission that match step1/3/4 of the NextSeg algorithm
* Correctly setting rescueRxt only if the rescue segment was actually sent.
* Correctly initializing rescueRxt/highRxt when entering SACK recovery.
* Correctly re-arming the timer only on retransmissions when SACK is in use
and not for every segment being sent as it was being done before.
* Copy over xmitTime and xmitCount on segment clone.
* Move writeNext along when skipping over SACKED segments. This is required
to prevent spurious retransmissions where we end up retransmitting data
that was never lost.
PiperOrigin-RevId: 310387671
08f4846ebe