This allows TLV serialization macros to read non-Option-wrapped
types but allow them to be missing, filling them in with the
provided default value as needed.

Trimmed-to-dust HTLCs are at risk of being burnt as miner fees
at anytime during their lifetime due to the broadcast of either
holder commitment transaction or counterparty's one.

To hedge against this risk, we introduce a new config setting
`max_balance_dust_htlc_msat`, with the initial value of
5_000_000 msat.

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.