add get_asset_markets

[oxarbitrage]

As a explorer/exchange I want get_asset_markets(asset_id) so that i can get a list of active markets in the last 24 hours for an asset.

This call should be added to asset_api (disabled by default) as it will have some performance issues.

Explorers and exchanges will use this call to populate active markets as: http://open-explorer.io/#/assets/1.3.3262 This is currently done client side looping once every X hours all assets, resulting in no uptodate results.

To be done after https://github.com/bitshares/bitshares-core/pull/1099

Impacts Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.

• [x] 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]

I think it's easier to implement this with market_history plugin, by making more use of market_ticker data, which should be more efficient.