[DS] Model iterators

[ajnavarro]

We need a way to transform model (repository,reference,commit,blob) -> Iterator[Row]
It should be an abstract class with all the generic code for all the models, and specific code to generate rows per each model.

This iterator should be able to process more than one repository in a lazy way.

[ajnavarro]

Assign to @ajnavarro . Create the first one to be able to split tasks.

[ajnavarro]

• RepositoryIterator: RepositoryIterator and ReferenceIterator implementations #20
• ReferenceIterator: RepositoryIterator and ReferenceIterator implementations #20
• CommitIterator: Pending
• BlobIterator: Pending. Mandatory have filtering logic implemented

[bzz]

CommitIterator #21
BlobIterator #25