Inconsistent sorting of call orders between matching against a limit order and a force settle order

[abitmore]

Call orders will be sorted then matched against other orders in two scenarios:

1. when being margin called
2. when being force settled

However, the sorting methods used in these two scenarios are different.

For margin calls, it's by_price (code):

const auto& call_price_index = call_index.indices().get<by_price>();

It's the call price when the short position was updated last time. It won't change.

For force settlements, it's by_collateral (code):

auto& call_index = get_index_type<call_order_index>().indices().get<by_collateral>();

It's the real-time collateral ratio of the call order. It changes every time when the order is partially filled.

The consequence: when a call order is partially filled, no matter if it's margin called or force settled, its real collateral ratio is increased, so it will have less chance to be matched against a force settlement order again; however, it will still be margin called at the initial call price.

Personally I think the by_collateral sorting is fairer and we should use it in both scenarios.

[pmconrad]

I agree that by_collateral is the better choice in both cases. I was actually surprised to find that the margin call code didn't update the call price after a partial fill when I wrote my recent explanation of margin calls.

OTOH I think this is a relatively minor issue. I'm therefore unsure whether this should be changed. In any case it's not a bugfix but a change of behaviour. IMO this means that the change requires shareholder approval.

[abitmore]

Done with #829.