[Data Liberation] Introduce WP_Entity_Reader_Iterator

[adamziel]

Introduces a new WP_Entity_Reader_Iterator( $entity_reader ) class to separate the iterator interface from the entity reader interface.

Rationale

Mixing the entity reader interface with the iterator interface is impractical, error-prone, and confusing:

• Iterator methods overlap with the EntityReader methods and it's confusing to have both next() and next_entity() with different semantice.
• The iterator interface expects the data to be ready on the first current() call, but the EntityReader interface assumes the first get_current_entity() call returns null until next_entity() is called
• Maintaining a copy&paste current(), next() etc. methods over a number of classes is a maintenance burden

Testing instructions

TBD. I haven't tested this PR yet.

There are no server-side unit tests for WP_Stream_Importer in trunk yet. AFAIR @zaerl you had something like that in your branch? It could come handy for testing this PR.

Aside of that, manual testing in the Data Liberation plugin is appropriate. Instructions TBD.

[zaerl]

I appreciated this approach here, good work. As already discussed, this is the best approach, and I think it will help with a trick I needed to solve in my PR.

AFAIR @zaerl you had something like that in your branch? It could come handy for testing this PR.

Let me try my new tests and see if they work without being touched in this context. If they do, we can merge this, and I will add them later with my PR.