Add VRF, and use VRF for proposer sortition, and random on AEV

[OnedgeLee]

Background

• Proposer selection is round-robin for now.
• Proposer selection portion is not proportional to its power, and have to be fixed for DPoS.
• Unpredictable proposer selection is recommended to prevent attack vector.
• With VRF, probabilistic power-proportional, unpredictable pseudo-random proposer selection can be accomplished.
• To generate secure, pseudo-random AEV, PreEvaluationHash is used. But there are chance to manipulate this value, by manipulating block parameters.
• With VRF, pre-determined pseudo-random seed can be made, and it cannot be manipulated.

Limitation

• Each validator can obtain expected random seed in advance, and this random seed can be real if validator is selected as a proposer.
• This can make validator to load transactions on block in selfish manner.