replace the RWMutex with a Mutex in the flow controller

[marten-seemann]

Obviously, the RW feature of the mutex was not used at all.

The mutex is needed because the Read go routines on multiple streams will call AddBytesRead. However, this is a write access. The only read access we have here is from the session go routine asking for flow control updates. An RW mutex however only makes sense if you have multiple go routines performing read access.

[codecov]

Codecov Report

Merging #2865 (3a06c18) into master (272229a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2865   +/-   ##
=======================================
  Coverage   85.67%   85.67%           
=======================================
  Files         134      134           
  Lines        9087     9087           
=======================================
  Hits         7785     7785           
  Misses        960      960           
  Partials      342      342           

Impacted Files	Coverage Δ
internal/flowcontrol/base_flow_controller.go	100.00% <ø> (ø)

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