tracking - LDK does not remove outgoing HTLC first, and does not provide preimage

when fulfilling an HTLC, LDK removes the incoming one first, which makes the payment unbalanced.

this was uncovered in !467 (merged) with the invoice_test functional test

Note: after the preimage is known to us, HTLCs are safe to settle (remove from commitment txs) in any order.

tracked by LDK in https://github.com/lightningdevkit/rust-lightning/issues/2356