Address performance regression in vote_cache::top

[clemahieu]

Revert c734caf which removed loop short-circuit checking for top voted-for elections.
Eliminate duplicate expression evaluation.
Use deque rather than vector for results to reduce backing reallocation.

[pwojcikdev]

I can't remember why I removed the by tally index, seems to make no sense.

EDIT:
Okay, so tally () is not a constant time function:

return std::accumulate (voters.begin (), voters.end (), nano::uint128_t{ 0 }, [] (auto sum, auto const & item) {
   return sum + item.weight;
});

Did you measure that the additional overhead on each insertion is worth the speedup during lookup? In any case, it's possible to implement some kind of smart tally caching to alleviate this issue and have both fast insertion and top lookup.