combine and optimize AgendasDB and VoteTracker

[buck54321]

trafficstars

These two parts of the agendas package have a lot of overlap. Recent issues have arisen from the DB side of things, and discussion has indicated a need to get as much info as possible live from dcrd rather than a local database or compiled-in network parameters.

The two package should be integrated into one and everything except historical vote counts should be retrieved directly from dcrd on startup.

[chappjc]

Can go in 5.0.1 or 5.1 unless there this is a show-stopper for 5.0.