Add database APIs to query tickets

[abitmore]

User Story

As a user I'd like some APIs to query tickets.

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 #2289.

[sschiessl-bcp]

Can you please elaborate what value of a ticket is and how it is calculated?

For next_type_downgrade_time I assume its the next time that the ticket changes its type (also upgrade)?
What happens on next_auto_update_time?

[abitmore]

@sschiessl-bcp see https://github.com/bitshares/bitshares-core/blob/test-5.1.0/libraries/chain/ticket_object.cpp#L164-L171, https://github.com/bitshares/bitshares-core/blob/test-5.1.0/libraries/chain/ticket_object.cpp#L118-L145 and etc. next_auto_update_time is for all updates and next_type_downgrade_time is only for downgrade.