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.
`Finally` doesn't take a `Func`, and I need to return the `Task` from the second call to `OnDisconnected`. Should I make a new `Finallly` helper that takes a `Func`?