ndt7: early exit

[stephen-soltesz]

After BBR stabilizes, the download connection may exit early.

[gfr10598]

I'd like to suggest that we make this an option selected by the client, with "legacy" as the default indefinitely.

1. aggressive-pacing-gain - terminates after the first positive crossing from <1 to >=1
2. conservative-pacing-gain - terminates after the second (or 3rd?) positive crossing
3. legacy - no early termination.

We might want to experiment with RTT based early termination as well. But the variance between 5th and 95th percentile MinRTT within the pacing-gain based bins is huge, so I'm guessing this won't work nearly as well.

However, there might be some benefit to waiting a minimum of 10 MinRTT before terminating. There seem to be a few ill-behaved tests where the unity gain crossing seems to come within just a couple MinRTTs, which seems wrong.

[gfr10598]

BTW, with first unity crossing, there is a roughly 8x reduction in server download network utilization!!!