Need a smarter way to clear cache in ezcomments. This is necessary especially when the commenting is busy.

[title changed from 'smart cache clearing' on Feb.19]

Steps to reproduce

1. Create a content with at least 10000 comments typically (using database insert ... select is the perfect way)
2. Add comment every 10 second
3. We can see the cache is cleared, so every time when we view comment from comment/view and change page typically, it's slow because comment cache is generated again.