remove long-warned deprecations (in prevision of 1.0)

[bagerard]

• Further to the deprecation warning, remove ability to use an unpacked list to Queryset.aggregate(*pipeline), a plain list must be provided instead Queryset.aggregate(pipeline), as it's closer to pymongo interface
• Further to the deprecation warning, remove full_response from QuerySet.modify as it wasn't supported with Pymongo 3+
• Fixed stacklevel of many warnings (to point places emitting the warning more accurately)

• optimize some bulk inserts in the tests (saved 10 seconds 🎉 )