Dust HTLCs on closed channels are always retryable

[TheBlueMatt]

At #1495 (comment) @wpaulino pointed out that we don't push IrrevocablyResolvedHTLCs onto our resolution list for dust HTLCs. This, however, results in dust HTLCs which are a part of an outbound payment being retryable on restart after they resolve as they are always included in the get_pending_outbound_htlcs return value. This isn't exactly likely, but it is theoretically possible that a user sends a dust-HTLC-payment, the channel closes, after the commitment transaction has reached six blocks the user retries the payment, which completes, then the user restarts and retries the payment again, which LDK will allow them to do because it loads the payment info back from the monitor.