Cosmetic filter cache: convert watermark-based pruning trigger to time-based pruning trigger

[gorhill]

Currently pruning is triggered using watermarks, with completely arbitrary watermark figures (75-100). The amount of cached CSS selector probably ends up being over the top, hence occupying memory pointlessly.

Using a time interval to trigger pruning makes more sense, given the cosmetic filter cache is useful mostly for when browsing web pages on a specific site withing a set time span. It's just a matter of picking after how long of not being used a cache entry should be considered obsolete.

[gorhill]

Actually a sort of combination of both approach would probably be optimal: prune obsolete entries at regular interval, except never throw out the remaining n most recently used entries.