Calculate LiNEAR staking reward fee from validator fees and beneficiary fees

[linguists]

Now the get_reward_fee_fraction() function return a fixed reward fee calculated from the weighted average fee of selected validators, and the total beneficiary fee: (100% - (100% - weighted average fee of validators) * (100% - total beneficiary fee))

However, the reward fee should be updated when the selected validator pool are changed. We can enhance this later by calculating the actual reward fee from the current fees of validators in contract, or ask manager to update the reward fee every epoch.