Subscribe to market tickers

**User Story**

As a core developer I think there are use cases to subscribe to market tickers.

Currently to get up-to-date market tickers, clients have to call `get_ticker` API repeatedly. If it's able to subscribe to the tickers, when a certain ticker has changed, the node will push new data to clients automatically.

@sschiessl-bcp @startailcoon what do you think?

**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

Start by reviewing the existing `get_ticker` API and the API portion of BitShares Core to understand how clients currently obtain market data. Define the subscription behavior and test cases, with done meaning subscribed clients receive new data when a ticker changes.

Written by the indexing model from the issue text.