Much like the database; peer contains id, address, channel contains
per-channel information. Where we create a channel, we always create
the peer too.
For the moment, peer->log and channel->log coexist side-by-side, to
reduce some of the churn.
Note that this changes the API to dev-forget-channel: if we have more
than one channel, we insist they specify the short-channel-id.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>