Call the preexisting OnDisconnected methods for all disconnects

- This means that the preexisting OnDisconnected can now be called due
  to a timeout caused by a client connecting to another server in scaleout.
In my opinion we should remove the OnDisconnect() overload entirely for the following reasons:
- we will start breaking people who are using both overloads - they suddenly will start getting duplicate notifications. This is an implicit breaking change you would only discover at runtime (can be hard to track)
- having two overloads can create a lot of confusion
- you don't need to worry if the order invoking is important
So, because we are making a breaking change anyway removing the OnDisconnect() overload seems better since it will be discovered when upgrading to the new version and will make the API clean.
(already talked to @halter73 about this)