Optimize MenuRenderer and make it configurable

[Jacob Rief]

One of the problems with the current MenuRenderer is, that it takes far too long to build the menus for large page trees. 

This has already been addressed by Stefan Foulis 3 years ago in this issue: https://github.com/divio/django-cms/issues/5802

Now, for my current project, we have ~93k pages and the current implementation of the MenuRenderer is unusable, because

after publishing one arbitrary page, it rebuilds the whole tree – this requires about 8 seconds. Even if the tree is cached, it takes

~1 second only to fetch all the menu nodes out of the cache.

Since in my case, only a small part of the menu has to be rebuild, it makes much more sense to rebuild the menu tree

partially, and to only cache that partial representation. Then, instead of fetching 93k nodes from the cache, only a few hundreds

have to be transferred. However, since such implementations may vary from project to project, it makes most sense to make the

MenuRenderer configurable through a settings variable (say CMS_MENU_RENDERER), so that smaller django-CMS installations

can rely on the default implementation.

I already started to implement such a change: https://github.com/jrief/django-cms/tree/fixes/5802-optimize-menu-rendering

and it looks very promising. The time to render CMS pages dropped from a minimum of >1 second to a maximum of about 200ms. This

means that my implementation is 5x faster without caching, than the default implementation with caching.

If the core maintainers of Django-CMS are interested in this change, please tell me, so that I can prepare my changes as a pull-request.

In any case, it would be hugely beneficial to the django-CMS ecosystem, because the current implementation is a show-stopper for

sites with more than a few thousand pages.

– Jacob