FEATURE: higher slack ratio out of the box

I upped the slack ratio for a few reasons

1. We render ucloaked anyway on first render,
   so cloaking really is not saving much
2. On mobile you don't get JS events so you need
   a lot more slack to minimize white screens
3. Vast majority of memory is used by object model,
   if we want to tame it we need to remove posts from stream

ember cloaking now supports high slack ratios without going into a tail spin
Remove min_posts_for_search_in_topic

no longer needed, we always search in topic
Minor optimisation avoid set in hot code path