conncheck: NOMINATION STUN attribute support
Attribute is proposed here: https://tools.ietf.org/html/draft-thatcher-ice-renomination-00 WebRTC supports this attribute: controlling side provides attribute value increased by one each time selected pair is changed: https://chromium.googlesource.com/external/webrtc/+/3c7d599750405bc734e1d5adbf1b54265b725a9d/p2p/base/p2ptransportchannel.cc#1821
See #49 (closed)
This patch handles the attribute by increasing the priority of the referenced pair to be greater than SELECTED pair's priority and forcing nomination, so referenced pair get selected.