Once we've finished streaming gossip from the first peer, we ask a
random peer (maybe the same one) for all short_channel_ids in the last
6 blocks from the latest channel we know about.
If this reveals new channels we didn't know about, we expand the probe
by a factor of 2 each time.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
The seeker starts by asking a peer (the first peer!) for all gossip
since a minute before the modified time of the gossip store.
This algorithm is enhanced in successive patches.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>