Roundup of issues from recent PRs

[guynir42]

Rounding up a bunch of semi-related issues into a single PR, to cleanup a bunch of open issues we collected when working on a set of recent PRs.

• Replace indexing of upstream_images for new_image and ref_image with a single ID of the ref image. Issue #178
• Replace the Image.header with Image.info and Image.raw_header with Image.header. Issue #187
• Remove limitations when making combined images, such as having different section ID or target. Issue #179
• Allow images to not have a section_id. Also part of Issue #179
• Add constraint on products that are unique with regards to image_id and provenance_id. Issue #160
• Make sure products have their own provenance_id as part of their filenames (to make them unique). Issue #146
• downstream_images as a lazy-loaded relationship on Image. Issue #151
• Remove recursive_merge() in favor of regular merge and the new merge_all() that runs on Image, SourceList and Reference, because it needs to take special care of the products (which are not relationships!).

[guynir42]

In addition, and I don't know why this happened, I started to make some minor adjustments to how we merge things in the data store. That caused a whole bunch of bugs that I have been running after for three days, touching half of our code.

[guynir42]

@rknop There are still a couple of test failures, related to object cleanup (not the actual logic of the tests). If you have time to review this, you can start, and I will fix the remaining minor issues tomorrow morning.

[guynir42]

@rknop I've addressed all comments. Please have another look and see if it is ok to merge (use the "see changes since last review" to save time). Thanks!