Correct OnchainEvent conf height for counterparty preimage claims

`OnchainEventEntry::confirmation_threshold`'s handling for
`OnchainEvent::HTLCSpendConfirmation` delays resolution of the
on-chain event until the CSV delay is met. However, this doesn't
make much sense for HTLC claims where our counterparty claimed the
HTLC with a preimage - the HTLC is resolved at 6 confirmations not
when the counterparty can get their money.

Thus, we should check for `$holder_tx` before setting the
`on_to_local_output_csv` field when handling HTLC transactions,
which we do here.

This also updates the code to match the documentation, which on
both `on_to_local_output_csv` and on the comment two lines above
this diff indicate that we should only include a value for claims
by us.

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -2961,7 +2961,7 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 										// wait for the CSV delay before dropping the HTLC from
 										// claimable balance if the claim was an HTLC-Success
 										// transaction.
-										on_to_local_output_csv: if accepted_preimage_claim {
+										on_to_local_output_csv: if accepted_preimage_claim && $holder_tx {
 											Some(self.on_holder_tx_csv) } else { None },
 									},
 								});