Local counter: Don't re-allocate maps in Go 1.21+

[VojtechVitek]

Yet another performance improvement for Go 1.21+. Reuse maps backing the sliding windows to avoid map re-allocation, resizing and garbage collection when shifting the windows every second (or minute).

Speeds up throughput by 2.67x and reduces memory allocations by 2x on Go 1.21+ 🚀

$ benchstat old.txt new.txt
goos: darwin
goarch: arm64
pkg: github.com/go-chi/httprate
               │   old.txt   │               new.txt               │
               │   sec/op    │   sec/op     vs base                │
LocalCounter-8   34.65m ± 3%   12.99m ± 2%  -62.52% (p=0.000 n=10)

               │   old.txt    │               new.txt                │
               │     B/op     │     B/op      vs base                │
LocalCounter-8   4.384Mi ± 0%   2.846Mi ± 0%  -35.07% (p=0.000 n=10)

               │   old.txt   │               new.txt               │
               │  allocs/op  │  allocs/op   vs base                │
LocalCounter-8   253.9k ± 0%   121.6k ± 0%  -52.12% (p=0.000 n=10)

(The CI runs Go 1.17, so we won't see any difference there.)

[github-actions]

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   19.67m ± 1%   19.53m ± 1%  ~ (p=0.143 n=10)

               │  master.txt  │             pr.txt             │
               │     B/op     │     B/op      vs base          │
LocalCounter-4   2.833Mi ± 0%   2.832Mi ± 0%  ~ (p=0.796 n=10)

               │ master.txt  │            pr.txt             │
               │  allocs/op  │  allocs/op   vs base          │
LocalCounter-4   121.5k ± 0%   121.4k ± 0%  ~ (p=0.810 n=10)