Match force-settlements with margin calls at normal margin call fill price

[abitmore]

User Story

As a user I'd like that force-settlements are matched with margin calls with conditions:

• no need to wait for the force-settlement delay when margin call appears
• match price is in favor of force-settle orders, i.e. at current_feed_price.settlement_price / (MSSR-MCFR)
  • if filling at the match price above would trigger a black swan event, fill at 100% CR price instead; after filled, if there are remaining debt in the margin call, trigger black swan event normally
• no force-settlement fee charged in this case
• the volume is not counted in the force-settlement volume per maintenance interval
• the matching takes place after processed limit (buy) orders with better price
• sell orders whose price are lower than margin calls won't be matched with force-settlements, because force-settlement are meant to improve overall collateral ratio.

Impacts
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.

• API (the application programming interface)
• Build (the build process or something prior to compiled code)
• CLI (the command line wallet)
• Deployment (the deployment process after building such as Docker, Travis, etc.)
• DEX (the Decentralized EXchange, market engine, etc.)
• P2P (the peer-to-peer network for transaction/block propagation)
• Performance (system or user efficiency, etc.)
• Protocol (the blockchain logic, consensus, validation, etc.)
• Security (the security of system or user data, etc.)
• UX (the User Experience)
• Other (please add below)

CORE TEAM TASK LIST

• Evaluate / Prioritize Feature Request
• Refine User Stories / Requirements
• Define Test Cases
• Design / Develop Solution
• Perform QA/Testing
• Update Documentation

[abitmore]

Done via #2489.