turn: add connection status to rfc6062 api

Add a connection status to turn_session on_peer_connection()
to notify when a peer disconnect.
Stop the peer connection on client side when the peer close the socket.

TurnTransport class changed to considere this new API and
transmit the information to user and closing peer IO.

Change-Id: I4997af9864829270876b6e6faddfa55da8ba7eb7
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>