fix race when stream.Read and CancelRead are called concurrently

[marten-seemann]

This is an alternative to #3240, moving the lock logic into the flow controller.
It also adds an integration test that is able to reliably reproduce the race condition.

Fixes #3239. Closes #3240.

[codecov]

Codecov Report

Merging #3241 (fbc30cd) into master (8906148) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3241      +/-   ##
==========================================
+ Coverage   85.43%   85.45%   +0.01%     
==========================================
  Files         133      133              
  Lines        9803     9806       +3     
==========================================
+ Hits         8375     8379       +4     
+ Misses       1053     1052       -1     
  Partials      375      375              

Impacted Files	Coverage Δ
internal/flowcontrol/stream_flow_controller.go	100.00% <100.00%> (ø)
internal/utils/rand.go	75.00% <0.00%> (+12.50%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8906148...fbc30cd. Read the comment docs.