Fix BSRM 2 Borrowing; Issue #3746

[litepresence]

Closes #3746

Use median_feed instead of current_feed in the case of a BSRM 2 individual settlement

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[sschiessl-bcp]

Since we have multiple black swan strategies, I think we need to detail out how each of them is used and which variables are set or not set and fix this method to work for all.

[sschiessl-bcp]

@abitmore since we have different black swan handling strategies, which variables are set in which case?

Is settlement_price and individual_settlement_debt both set at the same time, or either or?

[abitmore]

IMO, since the extractRawFeedPrice() function is used in different places for different purposes, we should be careful when updating code here to avoid introducing new bugs.

1. We should always use median_feeds.settlement_price when borrowing, to calculate CR and etc.
2. Use current_feed.settlement_price when displaying the "yellow" orders (margin calls) on the order book, placing limit order to buy into the margin calls, or requesting forced-settlements (note that it's not for the scenario when BSRM is 0 and a black swan event had happened), etc.
3. When BSRM is 1 or 2, median_feeds.settlement_price and current_feed.settlement_price can be different when there was a black swan event.

settlement_price is set when BSRM is 0 and a black swan event happens. It is used in this case for forced-settlements. Check the USD asset on chain.

individual_settlement_debt is set when BSRM is (2 or 3) and a black swan event happens. Check the CNY and SGD assets on chain.

[sschiessl-bcp]

Please add a comment how it relates, and why

[grctest]

If it has a positive individual_settlement_debt, then the current feed freezes, you need to switch to using the median feed to continue borrowing new individual settlement positions.