Look up node id from scid in OnionMessenger

When forwarding onion messages, the next node may be represented by a
short channel id instead of a node id. Parameterize OnionMessenger with
a NodeIdLookUp trait to find which node is the next hop. Implement the
trait for ChannelManager for forwarding to channel counterparties.

Also use this trait when advancing a blinded path one hop when the
sender is the introduction node.