WIP, closes T2724.
This diff was my starting work on solving the task, since quite a lot of changes
are required to solve it. What remains to be done:
- Add a buffer for data received by the iter_content() requests method to ensure the data flow is only cut after a complete line (and not randomly)
- Configure the mocking offline tests to use a stream API (similar to what is done in swh-web tests)
- Add a history/.status file similar to the existing history/by-date/.status
- Make sure by-date/ by-hash/ by-page/ do not rely on having complete history data