Make .get_storage() caching listing lazy

[ilongin]

This is follow up for PR #294 where listing is saved (cached) inside .from_storage() method itself which means it's not lazy unlike all the other DataChain steps.
We should think how to postpone caching / saving listing when all chain steps are applied.

Ideas:

1. Make some generic step that accepts function (closure) in DatasetQuery
2. Create .terminate() on DataChain class
3. ...