Changing block_interval is not handled correctly

**Bug Description**
The chain backend code assumes a constant block interval in various places, e. g. https://github.com/bitshares/bitshares-core/blob/3.3.1/libraries/chain/db_witness_schedule.cpp#L71 or https://github.com/bitshares/bitshares-core/blob/3.3.1/libraries/chain/db_witness_schedule.cpp#L79 .

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by this bug. 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.)
- [x] Protocol (the blockchain logic, consensus, validation, etc.)
- [x] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

**Steps To Reproduce**
Theoretical problem for now.

**Additional Context (optional)**
The impact of this problem still needs to be evaluated. There are no plans to change to block interval in the near future.

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Bug Report
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start with libraries/chain/db_witness_schedule.cpp at the referenced lines 71 and 79, then trace other places where the chain backend assumes a constant block interval. Evaluate which protocol and security paths are affected and define test cases for a changed interval; the issue is done when variable block intervals are handled correctly across the impacted paths.

Written by the indexing model from the issue text.