Simple collateralized P2P funding

[abitmore]

User Story

As a user I'd like a simple collateralized P2P funding feature exists in BitShares.

Key features:

• user A creates an offer to provide funds (asset X) with certain terms
  • what collateral (E.G. asset Y, maybe multiple asset types) to accept and what's the ratio
  • minimum amount for taking the offer if the offer allows partially taking
  • a deadline, when the funds should be returned
  • fee rate
  • who can take the offer (optional)
• user B takes the offer by collateralizing an amount of asset Y and opens a position
  • if user B returns the funds before the deadline, the position is closed normally;
  • if user B did not return the funds before the deadline, the position is closed too, and the collateral is sent to user A.

Note: not like BSIP 70, in this issue,

• the funds can be used with no limitation;
• there is no margin call.

See also: #2351 (comment)

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 #2479.