localCounter: Use RWMutex to speed up lookups

[github-actions[bot]]

Benchmark Results

goos: linux
goarch: amd64
pkg: github.com/go-chi/httprate
cpu: AMD EPYC 7763 64-Core Processor                
               │ master.txt  │               pr.txt                │
               │   sec/op    │   sec/op     vs base                │
LocalCounter-4   62.47m ± 1%   43.45m ± 2%  -30.44% (p=0.000 n=10)

               │  master.txt  │               pr.txt                │
               │     B/op     │     B/op      vs base               │
LocalCounter-4   4.363Mi ± 0%   4.378Mi ± 0%  +0.34% (p=0.011 n=10)

               │ master.txt  │               pr.txt               │
               │  allocs/op  │  allocs/op   vs base               │
LocalCounter-4   253.7k ± 0%   253.9k ± 0%  +0.07% (p=0.010 n=10)

[VojtechVitek]

-30.44% :)