Data Bucket Stream: fix reindexing

Hello,

please see the details of this MR in the commit messages.

The second patch breaks backwards compatibility. Since code that relied on the old semantics of __len__ may fail silently, I'd plan for the production instance I take care of, to not deploy it (or only temporarily when reindexing). However, I still think it's good to add it to upstream Wendelin, as any new project using Data Bucket Stream would benefit from it.

Best, Levin

/cc @Tyagov @paul.graydon @klaus

edit: tests pass