Lendroid Whalestreet

When updating `cachedRewardPerStake` in `updateRewards` modifier, the calculated value might be wrong. As example, if we assume that `updateRewards` was not called between epoch X and Y (X<Y) and where X belongs to a different rewards series than Y, the distributed reward will vary from the expected total reward distribution depending on how far apart are X and Y epochs. This issue is due to how `rewardPerStake` is implemented since `lastUpdateTime` was in a previous series with a higher or lower reward, and where the actual block.timestamp belongs to an ongoing series. The `rewardRate(currentEpoch())` will return the value of the new series and apply it to a period of the previous series distributing more or less reward than expected to the users.