Add new config setting `max_balance_dust_htlc_msat`

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.