Replace Tendermint's PoS to VRF-based Random Sampling

[torao]

Modify current Proposer selection by round-robin in the ValidatorSet to a random selection using VRF. Using #32, #40, #45

While this modification may result in dead code for many of the priority adjustments that ValidatorSet currently have, it leaves the priority adjustment code intact because of the way the Priority in the Tendermint PoS is used to weighted random selection.

[torao]

Investigation of modification

Where the ValidatorSet is used as a field

• RoundState: Validators, LastValidators
  • The Proposer field holds only the Address and Index in ValidatorInfo.
• HeightVoteSet: valSet
• VoteSet: valSet
• State: NextValidators, Validators, LastValidators

Where the ValidatorSet.Proposer is modified

• NewValidatorSet()
  • GenesisDoc.ValidatorHash() → only called in test code
  • Handshaker.ReplayBlocks()
  • state.MakeGenesisState()
  • client.provider.getValidatorSet()
  • lite.privKeys.ToValidators()
  • http.http.ValidatorSet()
• Copy()
  • CopyIncrementProposerPriority()
    • State.MakeGenesisState()
  • State.GetValidators()
  • State.enterNewRound()
• GetProposer()
  • StringIndented()
    • String() → general interface method
  • RoundState.RoundStateSample()
    • State.GetRoundStateSimpleJSON()
      • consensus.ConsensusState()
  • RoundState.NewRoundEvent()
    • State.enterNewRound()
  • State.enterPropose()
  • State.isProposer()
  • State.defaultSetProposal()
• IncrementProposerPriority()
  • CopyIncrementProposerPriority()
    • State.MakeGenesisState()
  • State.enterNewRound()
  • state.LoadValidators()
  • execution.updateState()
  • NewValidatorSet()
    • GenesisDoc.ValidatorHash() → only from test code

[torao]

I've fixed the Proposer election to be done by VRF. However, a few tasks and discussions remain to be done.

1. Some tests have been skipped. They assumed deterministic PoS behavior and assigned the role of Proposer to a fixed Validator, and that assumption is broken by the random selection by VRF. This is because it takes time to apply a (deterministic) randomly chosen Proposer in VRF to the same scenario.
2. The ProposerPriority, which is no longer needed due to random selection based on VotingPower, is retained. We leave it to consider how about to use ProposerPriority rather than VotingPower for weighting in order to prevent a succession of favorable states due to the large stake amount.