DRT: integrate DRT components to conversation

This patches avoid swarm members to connect to all devices of the
conversation by using a better routing mechanism.
The idea is to add a component called "DRT" that will store, like
a DHT, a routing table containing the known devices of the conversation.
The DRT is bootstraped with the list of the devices that already wrote
a message in the conversation, else with some other members that have
devices present (because lot of devices may never have written any message).
Moreover, because a user can import an old backup without the conversation
and without other devices present, we also try to connect to 2 other random
members because they may never connect if they are on an old back-up.

Once bootstraped, the DRT is used to decide to who a user should be
connected. And it will use approximately log(N) connections (instead
of N like before). So, when sending a message, a user will send to
connected peers (and some mobiles). Then, some peers in the conversation
will get the message, pull it, then announce to their nearest peers (and
some mobiles) until all peers got the new message. To avoid infinite loop,
we check if we already got the commit and if we have something to announce.

Finally, if somebody disconnect, this will cause the DRT to try to re-fill
its bucket and try to connect to a new device.

GitLab: #297
Change-Id: I137788c77219fe74287585882260547cc5628784