PoS rewards based on voting info

[tzemanovic]

Currently, in the spec for PoS reward distribution, validators receive reward proportional to only their stake. We need to take the block voting information into consideration to incentivize participation in block creation

[brentstone]

Voting info is received for every block, while rewards are distributed only at the end of an epoch when the last block is finalized. We also do not know how many blocks will be in the current epoch until the end of the epoch, but we will assume that each block will yield the same reward over the course of the epoch.

To accomplish this, a storage key that looks like #{PoS}/validator_set/consensus/block_reward_accumulator will hold a LazyMap<Address, Decimal> to store the accumulator value for each validator in the consensus set. At every epoch boundary, this LazyMap will be reset to be empty. For each block, the Decimal value will be incremented with the fraction of that block's reward received by each validator. At the end of the epoch, we calculate the fraction of the total inflation that goes to each validator by dividing the validator accumulator value by the number of blocks in the epoch. The reward products and other relevant objects in storage are then updated appropriately.