With anchor outputs, we can keep the commit tx feerate lower than the real
on-chain feerate. That means that when closing the channel, the resulting
fee will not necessarily be lower than the current commit tx fee, this
requirement doesn't make sense to be strict.

Both sides can optionally include a preferred fee range in their
`closing_signed`. This lets their peer know what fees they find acceptable
and should simplify negotiation.

When fee ranges are incompatible, disconnecting doesn't help much: upon
reconnection, the same `closing_signed` will be sent again.

It makes more sense instead to send a `warning` and keep the connection open.
The node operator should notice that warning and instruct its node to send
another `closing_signed` with a different fee range (if it agrees) or may
choose to ignore it and wait, or to force-close.

As discussed during the spec meeting, we can remove that requirement.

If we're disagreeing on the closing fee_range, no progress will happen,
so we force-close the channel after some time to unblock the funds.

And clarify introduction text.