Re-implement the index-to-storage method

[skoshx]

Right now Pentagon is implemented in a way where for every index (eg. primary, unique, index) we create a duplicate entry. It's not optimal especially when we have more data.

For instance, in the case where we want to fetch items only after a certain createdAt timestamp, currently we would need to fetch all the items, and then filter by createdAt. Instead, what we want is to be able to index by createdAt, only list the keys, and only after that fetch the actual items.

Denodata has a great implementation of how this should basically look like.