Improve vote tally performance (long term)

Job split from issue #803.

Vote tally performance affects maintenance block producing (https://github.com/bitshares/bitshares-core/issues/504) as well as overall sync/replay time.

Short term solution described in issue #803 and done in PR #982.

- [ ] Long term solution to be figured out.
- The mechanism used by Steem could be an option, but I think it only positively impacts maintenance block producing but probably negatively impacts overall sync/replay performance.

This is a sub-task of #982.

Start by reading issue #803 and PR #982 for the completed short-term approach, then review issue #504 for the maintenance-block impact. Compare the proposed Steem mechanism with sync and replay needs; the issue is done only when a long-term solution has been defined and implemented.

Written by the indexing model from the issue text.