Certain bitasset options should still be updatable after globally settled

[abitmore]

User Story

As an MPA owner I'd like to be able to update the bitasset options when my MPA is globally settled.
IMHO all options except the short_backing_asset should be updatable.

Note: if implemented, this comment should be removed:

bitshares-core/libraries/chain/db_market.cpp

Line 55 in 9c3f15e

* No more asset updates may be issued.

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)

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

[abitmore]

Done via #2506.