discovery: ensure port number uniqueness agent-wide
requested to merge bellet/libnice:use-different-port-numbers-for-every-local-host-candidates into master
The port number must be different for all local host candidates, not just in the same component, but across all components and all streams. A candidate ambiguity between a host local host and an identical server reflexive candidate have more unwanted consequences when it concerns two different components, because an inbound stun request may be associated to a wrong component.