Enforce `max_balance_dust_htlc_msat` at HTLC reception/forward

At `update_add_htlc()`/`send_htlc()`, we verify that the inbound/
outbound dust or the sum of both, on either sides of the link isn't
above new config setting `max_balance_dust_htlc_msat`.

A dust HTLC is hence defined as a trimmed-to-dust one, i.e including
the fee cost to publish its claiming transaction.