Allow update_fee sending during shutdown negotiation

[TheBlueMatt]

Currently if we've started shutdown we refuse to queue up update_fee messages, but we absolutely should, given they can be important if shutdown takes a while due to a stuck HTLC. Further, we may actually have an edge case where we send an update_fee as the very last thing in a channel - if there's an update_fee pending in the holding cell (and its the only thing in the holding cell) when we receive our peer's last RAA we may let the update_fee fly even though there are no remaining HTLCs, violating the BOLTs (cc lightning/bolts#970).