use MessageChannel for p2p protocol messages

The MessageChannel is used to exchange Jami protocol messages
between peers, similar to the SyncChannel but for communications
with contact devices instead of account devices.

It would ultimately be used instead of the SIP channel
for communication with peers, overcoming the pjsip message size limitation
and avoiding the overhead of the SIP message header.

The MessageChannel also allows transmission of raw binary data over the network,
which is not possible with a SIP transport.

Messages on the MessageChannel are serialized using msgpack.
It is used instead of the SIP transport when possible,
in a backward-compatible way, falling back to the SIP transport if a MessageChannel is not available, for instance when communicating with an
older Jami version.

Change-Id: I104c998486cbd5e520b1e3b92277613c84a3a305