Add a telemetry event on connect/disconnect

[v0idpwn]

This PR also moves the logic of notifying listeners to a telemetry handler.

Pros:

• We get telemetry events, which are more powerful than connection listeners, and more "standard" in the ecosystem.

Cons:

• The logic to support the listeners on top of the telemetry events is a bit ugly
• Two ways of doing the same thing.

I'm not sure if we should merge this. The purpose of the PR is to demonstrate how it could be done.

[josevalim]

Thanks but I think prefer the opposite route. Listeners are free, telemetry has a tiny cost. Can we have a built in listeners that pushes telemetry events?

[v0idpwn]

Sounds good, will send another PR with the alternative approach :)

[v0idpwn]

Closing this in favor of #311