Enhancements to peer backup restoration.

[rustyrussell]

1. We should only store latestscb if the timestamp is greater than the previous.
2. There should be a way to query the contents of latest scb.

Ideally, recovery looks like:

1. Connect to some node, get gossip.
2. Look for existing public channels. If they exist, connect to them.
3. Otherwise, connect (not too quickly!) to peers which offer recovery, in the hope one has our data.
4. When we have a peerbackup, look for more peers to connect to, based on what that says we have channels with.
5. Finally, when we have connected to all of those which are reachable, act on the latest scb (restorefrompeer)

showpeerbackup ? - gives raw backup hex, and details about it (esp. what peers!)
Also, restorefrompeer is undocumented, so we can rework that interface if we want.