sweep: remove all unconfirmed descendant transactions when a sweep conflicts

Before this commit, we we were trying to sweep an anchor output, and
that output was spent by someone else (not the sweeper), then we would
report this back to the original resolver (allowing it to be cleaned
up), and also remove the set of inputs spent by that transaction from
the set we need to sweep.

However, it's possible that if a user is spending unconfirmed outputs,
then the wallet is holding onto an invalid transaction, as the outputs
that were used as inputs have been double spent elsewhere.

In this commit, we fix this issue by recursively removing all descendant
transactions of our past sweeps that have an intersecting input set as
the spending transaction. In cases where a user spent an unconfirmed
output to funding a channel, and that output was a descendant of the now
swept anchor output, the funds will now properly be marked as available.

Fixes #6241

lnwallet/interface.go

@@ -346,6 +346,17 @@ type WalletController interface {
 	// is set. Labels must not be empty, and they are limited to 500 chars.
 	LabelTransaction(hash chainhash.Hash, label string, overwrite bool) error
 
+	// FetchTx attempts to fetch a transaction in the wallet's database
+	// identified by the passed transaction hash. If the transaction can't
+	// be found, then a nil pointer is returned.
+	FetchTx(chainhash.Hash) (*wire.MsgTx, error)
+
+	// RemoveDescendants attempts to remove any transaction from the
+	// wallet's tx store (that may be unconfirmed) that spends outputs
+	// created by the passed transaction. This remove propagates
+	// recursively down the chain of descendent transactions.
+	RemoveDescendants(*wire.MsgTx) error
+
 	// FundPsbt creates a fully populated PSBT packet that contains enough
 	// inputs to fund the outputs specified in the passed in packet with the
 	// specified fee rate. If there is change left, a change output from the