Fix incorrect reward index update

[0x-r4bbit]

After deploying some version of the staking system, we've uncovered a bug that rewards are calculated incorrectly when staking and unstaking.

The issue relies in the fact that, when the rewardsBalanceOf gets the newRewardIndex, the account.rewardIndex is possibly as big, which results a reward index delta of 0.

That in turn resets the rewards to 0 until some time passes.
We need to revisit that and ensure that we set the account.rewardIndex correctly when stake and unstake is called