Mark relayServerAddress and relayServerIP as optional in the signal proto, return the relay instance address and IP atomically from Manager.RelayInstanceAddress to avoid divergence across reconnections, and split the relay client constructor into NewClient and NewClientWithServerIP. Rename fallback terminology to server IP throughout.
In this PR, speed up the GRPC message processing, force the recreation of the ICE agent when getting a new, remote offer (do not wait for local STUN timeout).