Improve getNewEarningPower logic

[garyghayrat]

          We should make an issue to think abou this: right now the logic for the earning power calculation is duplicated in this method, and in the `getNewEarningPower` method, where it is intermingled with the qualification status boolean logic as well. There's probably a slightly cleaner way to implement this, but no need to bikeshed in this PR.

Originally posted by @apbendi in #24 (comment)

[garyghayrat]

How about something like:

function getEarningPower(uint256 _amountStaked, address, /* _staker */ address _delegatee)
    external
    view
    returns (uint256)
{
    if (isOracleStale()) {
        return _amountStaked;
    }

    return isDelegateeEligible(_delegatee) ? _amountStaked : 0;
}

function getNewEarningPower(
    uint256 _amountStaked,
    address, /* _staker */
    address _delegatee,
    uint256 /* _oldEarningPower */
) external view returns (uint256, bool) {
    if (isOracleStale()) {
        return (_amountStaked, true);
    }

    if (!isDelegateeEligible(_delegatee)) {
        bool isUpdateDelayElapsed = (timeOfIneligibility[_delegatee] + updateEligibilityDelay) <= block.timestamp;
        return (0, isUpdateDelayElapsed);
    }

    return (_amountStaked, true);
}

function isOracleStale() internal view returns (bool) {
    return block.timestamp - lastOracleUpdateTime > STALE_ORACLE_WINDOW;
}

function isDelegateeEligible(address _delegatee) internal view returns (bool) {
    return delegateeScores[_delegatee] >= delegateeEligibilityThresholdScore;
}