replace the RWMutex with a Mutex in the packet handler map

[marten-seemann]

Using an RWMutex only makes sense if reads handle in parallel. They don't in this case.

Locking and unlocking of a Mutex is significantly faster than of a Mutex:

BenchmarkMutex-16       100000000               11.8 ns/op             0 B/op          0 allocs/op
BenchmarkRWMutex-16     58777676                21.0 ns/op             0 B/op          0 allocs/op

[codecov]

Codecov Report

Merging #2864 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2864      +/-   ##
==========================================
- Coverage   85.67%   85.67%   -0.00%     
==========================================
  Files         134      134              
  Lines        9087     9086       -1     
==========================================
- Hits         7785     7784       -1     
  Misses        960      960              
  Partials      342      342              

Impacted Files	Coverage Δ
packet_handler_map.go	73.02% <100.00%> (-0.12%)	⬇️

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