Changing block_interval is not handled correctly

[pmconrad]

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.)
• Protocol (the blockchain logic, consensus, validation, etc.)
• 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