Add concept of connectors to cacheables

[bwees]

This PR allows a custom cache connector to replace the in-memory cache. This PR retains the default behavior of the library (in-memory cache) but allows you to specify a connector that implements CacheConnector. This will allow users to implement databases such as Redis or other KV stores as cache storage.

The in-memory cache has been broken into its own class that implements CacheConnector to simplify the code. This class is used by default if a connector is not specified.

[grischaerbe]

Hi @bwees and sorry for getting back to you 2 years after you posted this. in fact, it's the first time im ever seeing this PR and apparently it's not a me problem. I always saw a stray PR in the "number of open PRs" badge next to the "Pull Requests" Tab, it always showed 1, but it wasn't available in the actual list of Pull Requests.
Just today, when GH posted this notice about an "ongoing search incident", your PR surfaced, or at least that's the first time I was able to actually view it. Nevertheless I'm very sorry it took literal ages. The good news: In the meantime i implemented something that is very close and goes beyond your approach and requirements. V3 will support a multi-layer architecture which makes implementing custom cache buckets a breeze, i hope you like it. I'll leave this open in case you want to iterate on you approach 👍 Thanks!

[bwees]

No worries! The new implementation looks excellent.

Thanks!