netdoc: Do _something_ with ConsensusVoterInfo.

The ConsensusVoterInfo structs inside a consensus document capture information about who the voters believe that the voters are. Right now, the code does nothing with these structs except for parsing them.

We should probably have some kind of accesors for these structs, so that they can be inspected and/or cloned into new documents.

Conceivably, we should alert the user if the list of authorities in the consensus is other than the list that the software recognizes.

Conceivably, we should somehow check the list of signatures on the consensus against the list of voters. (This sounds like it would be a Security issues, but I think it isn't: if the authorities we believe in have signed a document that the client shouldn't believe, that's outside our threat model.)