Implement update_fee commitment dance and sending

[TheBlueMatt]

We currently implement update_fee by just updating the local fee, but it needs to do a full commitment dance to get updated (ie we need to store a previous one and a commitment sate of the new one). We also need to implement sending update_fees.

[TheBlueMatt]

At least the rec dance part of this may be a good learning excersize for someone if they want to learn a bit more about channel update dances and tracking. May be nontrivial and the sending part is probably a decent chunk more work.