[Rewards Streamer]: fix rewards calculation using only MPs accrued up to rewardIndex

[gravityblast]

In the current implementation the the rewards are calculated on a proportion between rewards and stakedToken + MPs.
The problem is that the now the calculation increment the MPs accrued by a user until the unstake moment.
It's correct to increment those MPs, but not all of them need to be taken into account in the reward calculation.
To fix that we need to use only the staked token and the MPs accrued until the reward.

[gravityblast]

closed with gravityblast/staking-rewards-streamer@a19b122