download ratelimit finetune

[icing]

When a download size is known and rate limiting is in effect, adjust the duration of each measurement step and its rate for maximum precision.

Since it is unpredictable how long the last bytes of a download will take, download speed can be thrown off if the "last bytes" are a significant amount of the total download. Make the "last bytes" small in comparison to the rest and "stretch" the rate limit intervals to accommodate the difference.

Fix ngtcp2 receive data acknowledgements to be based on a local window size tracking. This allows window updates controlled by rate limits.

Fix ratelimit wait time calculation to accommodate negative tokens.

Add ratelimit measurements in scorecard. Examples:

> python3 tests/http/scorecard.py -d --download-count=5 --download-sizes=128kb,512kb,1mb,10mb --download-parallel=1 --limit-rate=$speed h1

Download Speed, limit=256.000 KB/s, from httpd/2.4.66
   size            serial(5) [cpu/rss]
  128KB  257.401 KB/s, +0.5% [0.6%/12MB]
  512KB  257.676 KB/s, +0.7% [0.1%/12MB]
    1MB  256.890 KB/s, +0.3% [0.1%/12MB]
   10MB  256.147 KB/s, +0.1% [0.0%/12MB]

Download Speed, limit=500.000 KB/s, from httpd/2.4.66
   size            serial(5) [cpu/rss]
  128KB  501.302 KB/s, +0.3% [1.5%/12MB]
  512KB  502.791 KB/s, +0.6% [0.3%/12MB]
    1MB  501.455 KB/s, +0.3% [0.2%/12MB]
   10MB  500.431 KB/s, +0.1% [0.1%/12MB]

Download Speed, limit=800.000 KB/s, from httpd/2.4.66
   size            serial(5) [cpu/rss]
  128KB  801.391 KB/s, +0.2% [2.6%/12MB]
  512KB  802.930 KB/s, +0.4% [0.5%/12MB]
    1MB  801.564 KB/s, +0.2% [0.3%/12MB]
   10MB  800.357 KB/s, +0.0% [0.1%/12MB]

Download Speed, limit=1024.000 KB/s, from httpd/2.4.66
   size             serial(5) [cpu/rss]
  128KB  1025.588 KB/s, +0.2% [3.7%/12MB]
  512KB  1027.610 KB/s, +0.4% [0.6%/12MB]
    1MB  1024.773 KB/s, +0.1% [0.4%/12MB]
   10MB  1024.813 KB/s, +0.1% [0.1%/12MB]

Download Speed, limit=5120.000 KB/s, from httpd/2.4.66
   size             serial(5) [cpu/rss]
  128KB  4959.631 KB/s, -3.1% [25.9%/11MB]
  512KB  5048.880 KB/s, -1.4% [5.3%/12MB]
    1MB  5086.098 KB/s, -0.7% [2.5%/12MB]
   10MB  5114.312 KB/s, -0.1% [0.5%/12MB]

[icing]

And now with additional fixes that make h2/h3 work correctly:

Download Speed (h2), limit=800.000 KB/s, from httpd/2.4.66
   size            serial(2) [cpu/rss]
  128KB  792.025 KB/s, -1.0% [10.9%/11MB]
  512KB  801.773 KB/s, +0.2% [2.1%/12MB]
    1MB  801.091 KB/s, +0.1% [1.1%/12MB]
   10MB  800.302 KB/s, +0.0% [0.2%/12MB]

Download Speed (h3), limit=800.000 KB/s, from nghttpx: https:62578 [backend httpd/2.4.66]
   size            serial(2) [cpu/rss]
  128KB  792.184 KB/s, -1.0% [10.6%/12MB]
  512KB  800.064 KB/s, +0.0% [2.4%/12MB]
    1MB  800.376 KB/s, +0.0% [1.2%/12MB]
   10MB  800.174 KB/s, +0.0% [0.5%/12MB]

[bagder]

This is ripe for merge now, right?

[icing]

Yes, please.