[PoC] Push simple filter conditions into $lookup stage.#345

WaVEV wants to merge 2 commits intomongodb:mainmongodb/django-mongodb-backend:mainfrom

WaVEV:INTPYTHON-635-Slow-joins-for-ManyToMany-fieldsWaVEV/django-mongodb-backend:INTPYTHON-635-Slow-joins-for-ManyToMany-fieldsCopy head branch name to clipboard