discovery: use a single server reflexive and relay local candidate

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.