agent: add restart2() API to also reset local candidates
Calling nice_agent_restart2() or nice_agent_restart_stream2() will also reset the list of local candidates allowing for a subsequent gather phase to take into account the exposure of new network interfaces after the the previous gather phase has completed. The selected pairs or the previous connection check remain available, until a new set of pairs have been established.
With contributions from: Matthew Waters matthew@centricular.com
Supersedes !219