This configuration field lets node operators decide on the amount of dust
htlcs that can be in-flight in each channel.

In case the channel is force-closed, up to this amount may be lost in
miner fees.

When sending and receiving htlcs, we check whether they would overflow
our configured dust exposure, and fail them instantly if they do.

A large `update_fee` may overflow our dust exposure by removing from the
commit tx htlcs that were previously untrimmed.

Node operators can choose to automatically force-close when that happens,
to avoid risking losing large dust amounts to miner fees.

And add many tests for the corresponding scenarios.