Initial outline of how to incorporate this. Related to #338

New Types

• new type Evidence interface
• new DuplicateVoteEvidence struct contains two conflicting votes (note votes already contain val address and index) and the validator's public key (needed for verifying signature on old votes)
• possibly other types: invalid proposals, invalid votes, blocks with invalid txs, unjustified votes. Anything else ?
• Unjustified votes may be left for post-crisis recovery for now, until we move justification into the real-time consensus

Block Changes:

• new Evidence field in Block is a []types.Evidence and is usually empty
• new EvidenceHash data.Bytes in block header.

Consensus Changes:

• new Evidence field in RoundState is a []types.Evidence
• on ErrVoteConflictingVotes in tryAddVote, append to the evidence list
• in createProposalBlock, add all current rs.Evidence to the block via newBlock.AddEvidence(rs.Evidence)
• in finalizeCommit, remove from rs.Evidence anything that was included in the block

ABCI Changes:

• BeginBlock takes a beginBlockArgs containing a bitarray of validators who voted in the commit, and a list of validator addresses that have misbehaved (should this include types of misbehaviours?)
• Info response includes a HeightDiffForEvidence which is the number of heights back a validator should accept evidence for (to prevent spam).

Validation Changes:

• Need access to pubkey to validate the signature (include in evidence)

Storage Changes:

• Need to persist evidence to disk before its in a block so we dont lose it (perhaps an extra WAL entry in each finalizeCommit containing the remaining evidence in our ConsensusState ?)
• Need to index evidence so a given piece of evidence is only submitted once

Reactor Changes:

• evidence needs to be gossiped to peers quickly
• make a new reactor that copies the structure of the mempool reactor but just for gossiping evidence