When a new user joins the libp2p network they fetch previously-sent messages

[holmesworcester]

When a new user joins a channel, it connects to peers in such a way that it gets all previously-sent messages, or as many as possible/practical.

Possible approaches:

0. Git fetch to one peer
1. Git fetch to all known peers, in random order
2. Git fetch to some random peers
3. Broadcast an "I have joined" message, and fetch from the peer with the farthest-along HEAD?

I think 1 might be best, but it assumes all peers (or most peers) have all messages, which might not be true.