tor-chanmgr: update comments about selecting pending channels

After looking at more C-tor code, I think it's worth adding a TODO comment here so that later we re-evaluate how we wait on pending channels for new channel requests. The code I implemented is a bit different than what tor does, and I'm not sure which is better. I think it's okay to just add a TODO comment for now and we can revisit this when we come back to fill in all of the other TODOs.