This is generally verboten now, since there can be multiple. There are a
few exceptions:
1. We sometimes want to know if there are *any* active channels.
2. Some dev commands still take peer id when they mean channel_id.
3. We still allow peer id when it's fully determined.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: JSON-RPC: `close` by peer id will fail if there is more than one live channel (use `channel_id` or `short_channel_id` as id arg).