Clear out storage manager

Since the storage manager is a singleton (which works really well in production), there tends to be pollution in testing environment.

It would be nice to have a easy way to either clear out the singleton or replace it with a new storage manager.

[juanca]

Yes. Things to clean up:

• All models in all tracked collections
• All cached responses
• All expectations

Or better yet, we can expose a non-singleton storage manager for testing purposes...