GitLab

This should fix the use case of creating a stream 1, gather until finished
then creating another stream, gather its candidates, then setting the
remotes on stream 1 (which is already gathered) while stream 2 is still gathering.

The agent->local_addresses should not change when we gather_candidates, let
the user set it with nice_agent_add_local_address, if he does not, then we
should auto-discover interface *during* the gathering process, and not save it.
Currently, when we call gather_candidates, we discover interfaces and save it to
agent->local_addresses as if someone called the _add_local_address API... problem
is that if we have 2/3 interfaces, we do an audio call.. an interface disappears
(vmware closed/whatever) then we upgrade to a video call, then libnice will reuse
the old interface list instead of the current ones..

wtf.. should not set added to -1 since we continue adding other candidates and e want to schedule_next if one succeeded

We need to keep the GMainContext referenced so we can be sure it's
still alive when we remove the sources we've attached to it.