This commit adds basic syncing capabilities with peers if they are both
online.
It updates the work done on MVDS, but I decided to create the code in
status-go instead, since it's very tight to the application (similarly
the code that was the inspiration for mvds, bramble, is all tight
together at the database level).
I reused parts of the protobufs.
The flow is:
1) An OFFER message is sent periodically with a bunch of message-ids and
group-ids.
2) Anyone can REQUEST some of those messages if not present in their
database.
3) The peer will then send over those messages.
It's disabled by default, but I am planning to add a way to set up the
flags.
