Port code about use of the unlinked_index from Hive

**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

Review the two linked Hive merge requests and trace BitShares Core's block-push and unlinked_index handling. Confirm how a newly linked block should trigger additional linking of eligible out-of-order blocks; done means the restored behavior is implemented and its recovery and performance effects are validated.

Written by the indexing model from the issue text.