Refactor events publishing

Events subscription (/events API endpoint) attributes pseudo-unique identifiers to incoming subscribers: originally its host, then its subscription time. This is unecessary and leads to code complexity.

Introduce a JSONMessagePublisher to provide simple pub/sub mechanism for JSONMessage, and rely on this new type to publish events to all subscribed listeners. The original logic is kept for the 'since' and 'until' parameters, and for client disconnection handling.

Docker-DCO-1.1-Signed-off-by: Arnaud Porterie <icecrime@gmail.com> (github: icecrime)
Merge pull request #6199 from icecrime/event_subscription_refactoring

Event subscription refactoring