Experiment with ijson to support large files

[pbauer]

See #3

[fredvd]

Tested the PR with a 7.5Gb File.json full of base64 blobs, migrating from Plone 4 to Plone 5.2. It worked flawlessly, after I had raised Waitress it's max_request_body_size from default 1Gb to 10Gb. All other content were imported fine.

[mauritsvanrees]

Seems ready for merge.

[fredvd]

@pbauer I've merged this one because I need it present to simplify testing the other branch with the AT->DX export switches/modifiers. If you had another reason to delay this merge: Sorry ;-)

[pbauer]

that's fine.

Did you see the branch in #11? I just now rebased it on the current main.
I also have some code to migrate topics that uses the criteria-migrators from p.a.c. to migrate ATCriteria to querystrings.
I will put that in another branch since it is not ready for prime time and still brittle. I plan to migrate a portal with > 1500 ATTopics 🙄

[fredvd]

Yes I noticed that branch. I'm testing it this afternoon/evening on my current migration project. I've asked Maurits if he can take a look at the import from server feature for this migration. Uploading through the browser for the bulk contenttypes is too tricky and only feasible if the browser and site are on the same machine.

Collections are the most painful default content type to try to migrate. Skeleton in the closet to support these in c.exportimport are old style collections still present in some Plone 4 sites.