chanmgr: get rid of Arc around Channel

This is ultimately prompted by or a followup to !218 (closed)

It gets rid of Arc around Channel by making Channel cheap to clone. The Arcs weren't actually doing all that much because a circuit needs an owned Channel anyway (as I discovered in an abortive branch where I tried to make everything be Arc<Channel> and remove the clone impl).

I think I understand all this better now. I hope you like my taste re Display impl...