Mature positions aren't effected by negative interest

[jalextowle]

Problem

While working on #537, I noticed that negative interest isn't being applied correctly to shorts and longs in _applyCheckpoint. The current implementation contains this logic:

        // Close out the short positions with a maturity time equal to the latest checkpoint.
        // This ensures that shorts don't continue to collect free variable interest and
        // ensures that LP's can withdraw the proceeds of their side of the trade.
        uint256 maturedShortsAmount = _totalSupply[
            AssetId.encodeAssetId(AssetId.AssetIdPrefix.Short, _checkpointTime)
        ];
        if (maturedShortsAmount > 0) {
            uint256 shareProceeds = maturedShortsAmount.divDown(_sharePrice);
            uint256 flatFee = shareProceeds.mulDown(_flatFee);
            uint256 govFee = flatFee.mulDown(_governanceFee);

            // Add accrued governance fees to the totalGovernanceFeesAccrued in terms of shares
            _governanceFeesAccrued += govFee;

            // Increase shareProceeds by the flatFeeCharged, and less the govFee from the amount as it doesn't count
            // towards reserves. shareProceeds will only be used to update reserves, so its fine to take fees here.
            shareProceeds += flatFee - govFee;

            // Closing out shorts first helps with netting by ensuring the LP funds
            // that were netted with longs are back in the shareReserves before we
            // close out the longs.
            _applyCloseShort(
                maturedShortsAmount,
                0,
                shareProceeds,
                0,
                _checkpointTime,
                _sharePrice
            );
        }

        // Close out the long positions with a maturity time equal to the latest checkpoint.
        uint256 maturedLongsAmount = _totalSupply[
            AssetId.encodeAssetId(AssetId.AssetIdPrefix.Long, _checkpointTime)
        ];
        if (maturedLongsAmount > 0) {
            uint256 shareProceeds = maturedLongsAmount.divDown(_sharePrice);
            uint256 flatFee = shareProceeds.mulDown(_flatFee);
            uint256 govFee = flatFee.mulDown(_governanceFee);

            // Add accrued governance fees to the totalGovernanceFeesAccrued in terms of shares
            _governanceFeesAccrued += govFee;

            // Reduce shareProceeds by the flatFeeCharged, and less the govFee from the amount as it doesn't count
            // towards reserves. shareProceeds will only be used to update reserves, so its fine to take fees here.
            shareProceeds -= flatFee - govFee;

            _applyCloseLong(
                maturedLongsAmount,
                0,
                shareProceeds,
                0,
                _checkpointTime,
                _sharePrice
            );
        }

We normally adjust the share proceeds and reserves deltas to account for negative interest in the calculation functions. Since the logic is simpler in _applyCheckpoint, we just included the logic inline in _applyCheckpoint, which results in this issue.

Solution

We should apply the negative interest logic. Aside from this, we should refactor the code so that we have calculation methods.

[jrhea]

Pretty sure this was resolved in #593

[jalextowle]

Closed by #593.