filter_activitynames can be improved after dropping of global text cache

As a follow-on to MDL-43524 which removed global text caching, we need to examine the activity names filter and probably add MUC caching there to improve performance.

The activitynames filter is what puts links in texts to other activities in the same course.

Petr has raised that the fully correct way would be to use new events or the as-yet-unwritten hooks to invalidate the caches, but I think that might be too much to do before 2.7, so I want to see a quick solution here even if it may mean relying on less-perfect cache invalidation.

Tests showing performance before/after are required.