Return margin calls in `get_order_book` API

[abitmore]

User Story
It seems that get_order_book API was designed for retrieving a "combined" order book. It makes sense to add margin calls into the result if price of margin calls is in the range, because traders can place limit orders to buy into the margin calls.

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

[pmconrad]

Added DEX + UX impact. UI will want to adapt, e. g. display margin calls in yellow. @SSchiessl

[abitmore]

AFAIK reference UI doesn't use get_order_book API because the result of this API is rounded thus inaccurate, instead, it uses get_limit_orders and get_call_orders.