discovery: use a single server reflexive and relay local candidate

The rationale with this patch is that an agent running on a multihomed host have no gain to store multiple identical server reflexive and relay local candidates (those obtained by a stun bind request sent from each local interface). The stun requests that use the same default route are all expected to provide the same answer.

This patch should keep intact the ICE conncheck versatility, and drastically improve the conncheck time to completion, as the number of checked pairs is reduced.