agent: trickle ICE mode

agent: trickle ICE mode

In this mode, activated by setting "ice-trickle" property of a
NiceAgent to TRUE, a component without a successful connectivity check
will wait indefinitely for more remote candidates to come.

nice_agent_peer_candidate_gathering_done() must be called after
receiving the last remote candidate for the component to finally change
its state to NICE_COMPONENT_STATE_FAILED.