Find handler by address in Vert.x notifier unsubscribe if callback is not provided

For each channel we want to unsubscribe, the unsubscribe method expects to receive the callback/handler which was provided during subscription. So the callback mentioned in the doc is not a callback which notifies that the unsubscription has been completed.

It would be nice to document the above and add support to automatically find the handler/callback used in subscription if it is not provided.