natty exit on ICE callback loop

[atavism]

When connectivity tests fail on every candidate pair, and a working pair cannot be found, ICE falls back on TURN to relay traffic between peers. Since we've excluded this from the implementation, natty should exit after the entire pair list has been exhausted and multiple connection attempts have failed. Right now, natty sometimes enters an infinite loop of STUN 4-way handshakes and ends up considering ineligible pairs multiple times. I'm adding some extra checks to know when to properly exit.