Port code about use of the unlinked_index from Hive

[abitmore]

User Story

https://gitlab.syncad.com/hive/hive/-/merge_requests/226 and https://gitlab.syncad.com/hive/hive/-/merge_requests/227

After pushing a new block that links to the chain, check the unlinked_index to see if there are any more blocks that we are now able to link. This restores functionality that was accidentally removed from bitshares many years ago, and will help us recover from out-of-order blocks quickly

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)
• [x] 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)

Additional Context (optional) Add any other context about your request here.

CORE TEAM TASK LIST

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