agent: freeze the list of local ip addresses when gathering local candidates

With this patch, we establish the list of local ip addresses when gathering local candidates, and only there. We consider this list will remain constant during the lifetime of the agent, the same way we keep the list of local host candidates.

Reevaluating this list of ip addresses while the agent is running may have side effects when computing stun priority of pairs, whose a part of this priority is based on the ip address rank in this list. This rank may change, if network interfaces are added or removed while the agent is running.