Streamer: add BDP scaling

[alexpyattaev]

Problem

Streamer ignores RTT in all its calculations. This means that connections with high latency are heavily rate limited, much more than the stake amount would suggest. This happens before the actual stake-based stream throttling even has a change to kick in, and is thus very counterintuitive to the client.

A more principled version of #7954.

Some preliminary concepts:

• Sender can not have more than max_concurrent_streams worth of open streams at any time
• These together limit how many TXs can be “in flight” between client and server and not yet ACKd. Currently, these limits are computed based on stake. We need to compute them based on stake and RTT (as longer RTT means you need to have more things on the wire before you see an ACK).
• Beyond the limit mentioned above, there is an overall stream limit imposed by the code in stream_throttle.rs. That is out of scope of this PR, we are not modifying SWQOS here.

This mechanism is not intended as the actual rate limiter for complaint clients, just as a limit on network buffers. This will never allocate less bandwidth than what you'd get today.

Summary of Changes

• Assume that in the original code the RTT was expected to be ~50ms
• Scale max_concurrent_uni_streams with RTT if it is above 50ms

50 ms was chosen as a compromise between allowing more TPS and keeping impact small.

Impact

Before:

After:

Related: #8948 (which was reverted)

[t-nelson]

wasn't there a new test case before the helper was added and plumbing removed?

[alexpyattaev]

wasn't there a new test case before the helper was added and plumbing removed?

Yes. Should I bring it back? Added the unittests back in 228af98 but feel free to drop that commit.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.6%. Comparing base (6b94ee3) to head (228af98).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #10144     +/-   ##
=========================================
- Coverage    82.6%    82.6%   -0.1%     
=========================================
  Files         853      853             
  Lines      318566   318586     +20     
=========================================
- Hits       263382   263358     -24     
- Misses      55184    55228     +44     

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