CS-BOLD-021 Medium 5.5: Batches Can Be Used to Make Two Free Adjustments in a Row

[bingen]

Whenever a trove's interest rate is adjusted, it is checked when the trove's last adjustment took place. If it is less than INTEREST_RATE_ADJ_COOLDOWN ago, the upfrontfee for an early adjustment is charged.
However, the setBatchManagerAnnualInterestRate function only takes into account the last adjustment of the batch, not of the individual troves.
This means a user can do the following repeatedly:

1. Wait until their batch's (and trove's) last adjustment was more than INTEREST_RATE_ADJ_COOLDOWN ago.
2. Join another batch with a different interest rate, that was adjusted more than INTEREST_RATE_ADJ_COOLDOWN ago.
3. Adjust the interest rate of the new batch.
4. Wait until the batch adjustment was more than INTEREST_RATE_ADJ_COOLDOWN ago.
5. Join the first batch again (set the desired interest rate after step 1.)
6. Adjust the interest rate of the first batch.

In this way, the user is able to adjust their interest rate twice in a row (steps 2./3., as well as steps 5./6.)
without paying the upfrontfee. Additionally, they only need to wait for the INTEREST_RATE_ADJ_COOLDOWN to pass once per pair of adjustments. This is because the trove gets to take advantage of the cooldown on both batches passing simultaneously.