Reduce Mux logging on Close

[sukunrt]

This removes warning logs like:

Warning: mux: no endpoint for packet starting with 21

when closing the connection.

Replace RWMutex with Mutex because there's only 1 goroutine reading.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.92%. Comparing base (e6a86e7) to head (7869792).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2901      +/-   ##
==========================================
- Coverage   78.94%   78.92%   -0.03%     
==========================================
  Files          89       89              
  Lines        8488     8489       +1     
==========================================
- Hits         6701     6700       -1     
- Misses       1299     1301       +2     
  Partials      488      488              

Flag	Coverage Δ
go	80.48% <100.00%> (-0.03%)	⬇️
wasm	65.12% <100.00%> (+0.01%)	⬆️

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[edaniels]

Looks great