pack: improve balance between votes and non-votes

[ptaffet-jump]

Even if all the most lucrative transactions are votes, we don't want to fill the priority queue with votes because we'll hit the vote cost per block limit and have nothing else to schedule.

   Case             New Vote                 New Non-vote
   Votes < 25%   Replace worst non-vote    If better, replace worst
                 with it                   non-vote with it
   Votes > 75%   If better, replace        Replace worst vote with
                 worst vote with it        it
   Else          If better, replace worse of worst non-vote and
                 worst vote                                      

For example, if we're receiving very few votes, we'll use the entire priority queue for non-votes, but when we get a vote, we'll automatically insert it, even if it's less lucrative than one of the non-votes.