EPOCHS: Decouple reward payment from end of epoch

[joshuahannan]

Issue To Be Solved

Need to be able to calculate rewards for an epoch and pay at separate times

Suggest A Solution

• Change payRewards to calculateRewards and have it return a dictionary mapping nodes and delegators to their reward amounts
• Add a payRewards function to the the epoch contract to pay rewards to all the nodes from the previous epoch
• Add a function to scale the rewards of a staker if needed for slashing an such
• Add a function to scale all the rewards of all stakers in case the epoch was short or long
• Only store staking information for the previous epoch