Experiment to free significant amount of RAM by getting rid of information never used after HF19

When I was cleaning vote related code before work towards removing of vote edit penalty, I've noticed that some data is updated but never used. When that was removed, it turned out that some fields that reside in comment_cashout_object can be dropped entirely after HF17 (later it turned out that it was not entirely true, that at least one field had to be kept for a week after HF17, so I pushed the change to HF19). However most profitable change was the ability to drop link to root post from comment_object (since these are most populous) and related index. That change gives over 3GB at 40 millionth block and it is only going to give more with more comments.