Improve performance of account history wallet APIs by using the block_time field in operation_history_object

[abitmore]

User Story

We are adding a block_time field in the operation_history_object class in #2642.

For better performance, in CLI wallet we should use that field instead of calling get_block_header to get the timestamps.

I'm adding this to the 7.0 milestone to avoid a compatibility issue, although technically it is not a hard fork change. Because API nodes running older versions won't return the new field, connecting a new CLI wallet to an old API node won't work (see #1307 for an example). Making the change along with a protocol upgrade release will guarantee that all API nodes will be running the new version.

Other client applications should update similarly.

• BitShares-UI
• BitShares Mobile
• others

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)

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 #2644.