Exhaustive search mode: implement exhaustive search logic in backend

[philipperolet]

NOT READY TO BE STARTED; please read first Current design discussions here Design doc here

This card takes care of the logic to perform an exhaustive search:

• gathering all docs from a relative timeframe rather than doing a semantic search query
• checking for context overflow, returning only docs fitting in context in antechronological order, and a timestamp up to which the documents could be gathered

[spolu]

@philipperolet I belive this task should not really exist. We already support this. We just miss a "has_more" flag in the response when query: null