The nomination of a pair having such a local candidate breaks SfB when
the libnice agent is behind a nat that does not do port mapping
randomization. In that case a server reflexive local candidate usually
lead to a nominated pair.

The guess made here from observing this behavior is that, it is valid to
discover and signal these local server reflexive candidates to our peer,
but they should be removed from our local candidates list thereafter, so
they do not contribute to build a valid and *even worse* a nominated
pair with the type server-reflexive. They do not appear in the conncheck
list per design anyway.

Instead, the same candidate is discovered again later during the
conncheck, with a peer-reflexive type this time, and with that type, it
just works.

Closes #90