perf: use map for sting-based stores

[gurgunday]

When an object is used as a map and has string-based keys, Map's performance will be superior

For this repo, not sure how to test it but here's some related discussion and benchmarks in kibertoad/toad-cache#40

And a V8 engineer explaining why: https://stackoverflow.com/questions/62350146/why-map-manipulation-is-much-slower-than-object-in-javascript-v8-for-integ/62351925#62351925

[mcollina]

Did you run the benchmarks? Where there any improvements?

[gurgunday]

I will post a benchmark if I can

[gurgunday]

Yeah no significant improvement here