Dynamic keys: create a set behind-the-scenes with all the dynamic keys names

[Radagaisus]

Great for iterating over dynamic keys.

Assuming:

@zset 'books', key: (author) -> "books:#{author}"

Allow developers to query a books_keys for all the dynamic keys that were generated. Or, .dynamic_keys_for('books'), or .keys_for('books'), or .keys('books'), or other options.

The end-developers will manually need to remove items from these lists, as there's no easy (or necessary) way to automatically do this for them.