Limit inbound fee updates by dust exposure instead of our estimator

Limit inbound fee updates by dust exposure instead of our estimator

Inbound fee udpates are rather broken in lightning as they can
impact the non-fundee despite the funder paying the fee, but only
in the dust exposure it places on the fundee.

At least lnd is fairly aggressively high in their (non-anchor) fee
estimation, running the risk of force-closure. Further, because we
relied on a fee estimator we don't have full control over, we
were assuming our users' fees are particularly conservative, and
thus were at a lot of risk to force-closures.

This converts our fee limiting to use an absurd upper bound,
focusing on whether we are over-exposed to in-flight dust when we
receive an update_fee.