mitm: improve parallelism when generating certificates

[mmatczuk]

The current mitm implementation uses RWMutex to protect the certificate cache. This causes stalls when multiple certificates are generated at the same time - which happens fairly often if the cache is cold.

I suggest to use sync.Map instead of RWMutex protected map, and allow parallel generation of multiple certificates by sharding.

The worker pool pattern will not work here because we want to avoid generating certificates for the same host twice. Instead we will:

• Add NxCPU mutexes - the N value is to be evaluated by a benchmark
• Calculate hostname hash using hash/maphash the exported hash map hash func
• Lock appropriate mutex for the cert generation

Also, this requires less changes than adding a worker.

[mmatczuk]

This seems not to be an issue. The thing is that the certificates are generated in parallel and can be overwritten in cache using RWMutex.

I have implemented the fix and it's marginally slower in the benchmark

benchmark            old ns/op     new ns/op     delta
BenchmarkCert-8      18650793      19137203      +2.61%
BenchmarkCert2-8     2371911       2375468       +0.15%

https://github.com/saucelabs/forwarder/compare/mmt/parallel_mkcert?expand=1

Maybe we'd go back to it but right now I'm closing the issue.