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.

server.go

@@ -1056,7 +1056,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		FeeEstimator:   cc.FeeEstimator,
 		GenSweepScript: newSweepPkScriptGen(cc.Wallet),
 		Signer:         cc.Wallet.Cfg.Signer,
-		Wallet:         cc.Wallet,
+		Wallet:         newSweeperWallet(cc.Wallet),
 		NewBatchTimer: func() <-chan time.Time {
 			return time.NewTimer(cfg.Sweeper.BatchWindowDuration).C
 		},