Better concurrency support

[ambyjkl]

Hello,
I want to use adblock-rust in a setup where concurrency would be beneficial. Although disabling object-pooling and unsync-regex-caching is enough to make Engine Send + Sync, there is still work to be done to actually make concurrent usage efficient. Hence, I want to contribute towards this goal, here is what I'm planing to do:

• Use a thread-local variable for the request_tokens vector
  • I believe this solution might even outperform the object pooling implementation, and hence eliminate the need for the lifeguard dependency
• Use a concurrent cache rather than a mutexed hashmap
  • While I have not used any concurrent cache other than redis (dashmap doesn't count as it doesn't have eviction support) in previous projects, after a bit of research, mini-moka (https://crates.io/crates/mini-moka) in particular caught my attention, since it has all the features we need while also providing a non-thread-safe implementation for the single-threaded case, so there can be high code reuse, while also being the sibling of moka which is battle-tested in production on crates.io itself

Please let me know your thoughts

[ambyjkl]

On second thought, time-based eviction is probably not the best idea; we don't lose much by leaving the compiled regex in the cache, since recompiling can be expensive. Also, Instant is not available on wasm32-unknown-unknown. Considering this, I think quick_cache (https://crates.io/crates/quick_cache/0.6.5) is a better fit.