agent: fix server-reflexive candidates with oc2007r2

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 (closed)