refactor: encapsulate lru cache in ratelimiter package

What does this MR do?

The idea is to separate the main package from the internal implementation of the ratelimiter package. We now just supply config options, and don't care about the lru package.

Part of #630 (closed)

TODO

• I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines