Overhaul stats: Allow multiple events listeners (subscribers to events)

[josecelano]

Relates to: #1341 (comment)

In order to start collecting metrics per socket address (segregate metrics per tracker service running on a different socket) we need to add more than one listener. There will be one listener per service that will update the metrics for that service.

Right now, the "factory" stats function creates both the event sender and the listener. We need to be able to:

• Create one sender that can be shared.
• Be able to create many listeners (subscribe listeners to events). One listener per socket.

For the time being we will keep the logic inside the statistics module. However, after finishing the overhaul of the stats we will decouple events from stats. In the future events might be used for other purposes, not only collecting metrics.

cc @da2ce7

[josecelano]

All sub-issues implemented.