sweep: remove conflicted sweep txns from the rebroadcaster

In this commit, we an existing gap in our rebroadcast handling logic. As
is, if we're trying to sweep a transaction and a conflicting transaction
is mined (timeout lands on chain, anchor swept), then we'll continue to
try to rebroadcast the tx in the background.

To resolve this, we give the sweeper a new closure function that it can
use to mark conflicted transactions as no longer requiring rebroadcast.

sweep/interface.go

@@ -35,4 +35,10 @@ type Wallet interface {
 	// hash passed in. If the transaction can't be found then a nil
 	// transaction pointer is returned.
 	FetchTx(chainhash.Hash) (*wire.MsgTx, error)
+
+	// CancelRebroadcast is used to inform the rebroadcaster sub-system
+	// that it no longer needs to try to rebroadcast a transaction. This is
+	// used to ensure that invalid transactions (inputs spent) aren't
+	// retried in the background.
+	CancelRebroadcast(tx chainhash.Hash)
 }