Handle all the scenarios for commitment discounts

[flanakin]

Description

Review all commitment-based discount scenarios to ensure we're being explicit about expectations in the data.

• Purchases
• Exchanges
• Returns
• Transfers between accounts
• Amortization logic/calculation
• etc.

Proposed approach

Review each scenario and ensure they are documented adequately.

[jpradocueva]

Check if it can be merged with other items. Classified as Overall Spec by the Maintainers on the May 24 call

[kk09v]

Notes on a how I would think about a commitment exchange.

Commitment exchange:

• One transaction zero-ing out the old commitment (negative billedcost, zero effectivecost)
• One transaction for the for the purchase of the new commitment (positive billedcost, zero effectivecost)

As I understand it, this is what happens in the exchange, with the requirement that the new purchase is greater than the one being exchanged.

If the new commitment is less than what's being exchanged:

• One transaction zero-ing out old commitment being credited toward the new commitment (negative billedcost, zero effectivecost)
• One transaction reporting the remainder (above the new purchase) as unused commitment (zero billedcost, positive effectivecost)
• One transaction for the for the purchase of the new commitment (positive billedcost, zero effectivecost)