Rebase reward

[stalker474]

Closes #51

The issue with this implementation is that we can potentially mint any amount of EEFI so it's a security issue and a decentralization issue. I suggest we add a limit to the reward amount.

[davoice-321]

Agree with setting a max rebase reward limit, in three steps:

1. Create a new state variable:

uint256 private maxRebaseReward;

2. Set maxRebaseReward: == 2 EEFI in constructor

3. Update setRebaseReward with require statement (as below):

function setRebaseReward(uint256 newRebaseReward) external onlyOwner() { require(newRebaseReward <= maxRebaseReward, "Rebase reward cannot be higher than maxRebaseReward."); rebaseCallerReward = newRebaseReward; emit RebaseRewardChanged(newRebaseReward); }