Port BIP0009 over from btcd. The following is an overview of the
changes:
- Add new configuration options to the chaincfg package which allows the
rule deployments to be defined per chain
- Implement code to calculate the threshold state as required by voting
- Use threshold state caches that are stored to the database in order
to accelerate startup time
- Remove caches that are invalid due to definition changes in the
params including additions, deletions, and changes to existing
entries
- Verify that PoW and PoS majorities have been reached.
- Add tests for new error type
- Add tests for threshold state
- Deployments are per stake version.
- Add human readable vote/choice infrastructure.
- Add RPC command to interrogate vote status (getvoteinfo).
