feature: Cache for the file handler to avoid stat()ing on every request

[MalloZup]

This issue comes from readme.md.

@krig you migh add some infos details about it. 🚀

[krig]

The idea is to add some kind of bounded in-memory cache for static files, to avoid having to go touch the filesystem for the file on every request. Especially on a file system which updates access time, it should add a bit of performance. On the other hand, caches are complicated and it can be confusing if the api server serves a cached copy instead of a file that has changed on disk, so some care has to be taken.