Fix push broker events being fired multiple times for a push service with multiple registered notification types

Registering multiple notification types for a push service with the broker (e.g. for Windows & Windows Phone) caused the events to be triggered multiple times, once for each registration.

This patch modifies the push broker to only wire up events once for any given push service.

Also fixes a possible null reference exception with GetReferences when passing an applicationId if there has previously been a registration with a null applicationId.