[rewrite] Rewrite 2.x from scratch

[rennokki]

Working with the 2.x seemed a difficult job:

• Replication interfaces and Channel Managers ping-ponged with the ways of storing connections and statistics for current live server
• No TDD-driven test, some old tests lacked integration tests at all
• The WebSocket Server's HTTP API endpoints lacked support for horizontal autoscaling
• Lack of consistency (some methods returned ints, some others promises; now all promises-based methods are returning fulfilled promises)
• Because of the TDD-driven aspect, we reach better coverage and on-point tests
• Simpler project tree

Expect a doc update & PR rewrite with the changes to take into consideration when migrating from 1.x to (now beta) 2.x