Delegation Fees + unit tests

[cmalfesi]

Delegation Fee

• Keep track of delegation fee variation in order to be able to determine the fee to use when a epoch reward is being claimed

struct DFeeData {
    uint256 fromEpoch;
    uint256 fee;
    bool applied;
}

DFeeData[] public delegationFees;

event CommitNewFees(uint256 deadline, uint256 feeRate);
event NewFees(uint256 fromEpoch, uint256 feeRate);

Determine Epoch Fee

• Iterate delegationFees backwards, take first where fromEpoch is lower or equal to the given epoch

Commit new fee

• Only 1 pending fee at any given time
• If at the moment of committing a new fee there is a delegation fee pending to be applied:
  • The pending fee still not able to applied, due to deadline not reached, the new fee will replace the pending one as the new pending
  • The pending fee can be applied then mark as applied and add the new one as pending. Emit NewFees event
• Emit CommitedNewFee event

Apply Fee

• Only mark a fee as applied if current epoch is greater or equal to fromEpoch
• Mark a fee as applied when a new one is committed an the pending one can be applied
• Mark a fee as applied when claiming reward for an epoch which needs to use the pending fee