Refactoring

[StarpTech]

• Use Nock and got instead axios and axios-mock-adapter
• Improve tests
• Use mongodb ttl for subscription lease feature
• Simplify implementation in order to start with a basic but solid foundation
• Remove rudimentary websocket implementation
• Use retry mechanism of got package
• Use p-map to distribute content requests with concurrency limit
• Use stream approach everywhere
• Use async / await
• Add support for Node.js 8/9/10