feat: implement iteration over cache elements

[ilyatotl]

Description

Often, Ristretto is used not only as a cache before databases, but also as a complete in-memory storage. And there are cases when you need to do something with all the elements in the cache (for example, completely unload them and send them somewhere). However, in the current implementation, this is impossible. For this, the Iter function was implemented, which goes through all the cache elements and calls a callback function for each one of them.

P.S. In addition, when running the tests, it was noticed that they were running for quite a long time, so they were parallelized, which significantly accelerated their work.

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to
  this PR
• Tests added for new functionality, or regression tests for bug fixes added as applicable

[mangalaman93]

Thank you for the PR. Do we need to change the API here at all for having iteration function? If not, I'd recommend breaking this PR into two. It would be easy to get one in that does not change the API, and only adds a new API. We can later on review the other one if and when we plan for a major release.