multiplexed_socket: add ping/pong logic

Detecting that a socket is down via a keep alive can take time, moreover
if a relay is used, because the keep alive can continue to work even if the
connection is half closed.
In this patch, we add a mechanism where a beacon is sent over the multiplexed
socket to know if the peer can answer. This message can be used to detect
if a socket is usable and is sent whenever the user starts a call or that
a connectivityChanged() occurs.
Moreover, the multiplexedsocket can now sends a version to enable features
and if a bad packet on the control socket comes, the socket is not stopped.

GitLab: #542
Change-Id: Ia66d8d7f9b66bba02927f4ea41c21ef27089bceb