Credit deal auto-renewal feature

[abitmore]

User Story

As a borrower, I would like that my debt can be (optionally) renewed automatically when it comes due.

While it looks similar to the auto-repayment feature described in #2595, it is actually much more complicated.

These things are likely to happen when a credit deal comes due:

• The original offer may have been disabled already;
• The original offer may have changed, e.g. requires more collateral, or shorter repay period, or higher fee rate, etc.

A good auto-renewal feature is not easy to design.

• A simple "renew no matter what changes" feature is easy to implement but likely won't be used, because lenders may take advantage of it by changing conditions at the last minute.
• A simple "renew if and only if condition has not changed" feature is not hard to implement, and might be used often, but may be rarely applicable and may frustrate users with expired deals and lost collateral.
• In any case, if a borrower decides to auto-renew when certain (whatever) conditions are met, the info must be disclosed on the blockchain before the debt is due, as a result, the lender will know about it and may take advantage of it, which puts the borrower at a disadvantage.

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