Turns out, our getnextvalidators implementation already works the way
getcandidates is supposed to work, but original getnextvalidators works a bit
differently. It only returns validators, it doesn't return Active flag (all
of them are active) and it represents votes as a number. So for the maximum
compatibility:
* drop non-validator keys from getnextvalidators server-side
* drop Active flag client-side (sorry, it doesn't exist)
* allow unmarshalling old answers along with the new one
This technically breaks `query candidates` CLI command, but it'll be fixed
when getcandidates are to be introduced.
