To reduce the surface area of amount of a channel balance that can be
eaten up as htlc dust, we introduce a new config
'--max-dust-htlc-exposure-msat', which sets the max amount that any
channel's balance can be added as dust
Changelog-Added: config: new option --max-dust-htlc-exposure-msat, which limits the total amount of sats to be allowed as dust on a channel