Removing use of gorilla/schema : less sexy, but reduce data injection
via hand-crafted requests. Bonus: performance should increase a bit as
no reflection. Still clumsy around data edition.
Adding a Owns(user, data) bool to to store interface.
Adding a cache in the sql store for ownership to implement Owns().

This allows to keep ownership checking in model.go and not
within sql requests.

Maybe a bit premature to add a cache..