Improve memory handling

[zenangst]

This PR aims to improve the memory handling when using Cache.

Before, we shared the same variable for DiskStorage and MemoryStorage which ended up not being what we want. And here is why:

DiskStorage’s limit is based on actual disk space where as MemoryStorage is based on the amount of objects that you want to keep in memory.

So this PR adds another variable to the configuration called maxObjects, it defaults to 0 so that it does not break the current implementations but the components using Cache now gain more fine-grained control over both storages.

Also, internally inside didSet in MemoryStorage but it was never called because it was only called inside init.

As and added bonus, this PR also adds SwiftLint and fixes some of the warnings that it introduces.

With Imaginary this change made quiet an impact on memory usage;

Before

### After

[mention-bot]

By analyzing the blame information on this pull request, we identified @vadymmarkov and @attila-at-hyper to be potential reviewers

[onmyway133]

Sometimes I feel the documentation is a bit unnecessary. I can see that by looking at the function signature

[zenangst]

I agree, they are a bit obvious but where would you draw the line? Also, the linter forces you to add documentation.