Revamp event handling

[nishanttotla]

This is a high-level issue to track all the work required to move events in Swarm to use github.com/docker/go-events, which is a well tested and robust library that meets all our needs. List of items:

• Move events for external subscribers to use go-events for Swarm clusters (Moving event handling for external subscribers to use go-events #2747)
• Refactor old event handling code (Refactor and move event handling code into the cluster package #2752)
• Collect events continuously from an Engine (collect events continuously from an engine #2717, Collect events continuously from an engine #2854)