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.

sweeper_wallet.go

@@ -0,0 +1,25 @@
+package lnd
+
+import (
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/lightningnetwork/lnd/lnwallet"
+)
+
+// sweeperWallet is a wrapper around the LightningWallet that implements the
+// sweeper's Wallet interface.
+type sweeperWallet struct {
+	*lnwallet.LightningWallet
+}
+
+// newSweeperWallet creates a new sweeper wallet from the given
+// LightningWallet.
+func newSweeperWallet(w *lnwallet.LightningWallet) *sweeperWallet {
+	return &sweeperWallet{
+		LightningWallet: w,
+	}
+}
+
+// CancelRebroadcast cancels the rebroadcast of the given transaction.
+func (s *sweeperWallet) CancelRebroadcast(txid chainhash.Hash) {
+	s.Cfg.Rebroadcaster.MarkAsConfirmed(txid)
+}