multi: query mempool spend when a new input is received

This commit changes how a new input sweep request is handled - now we
will query the mempool and see if it's already been spent. If so, we'll
update its state as we may need to RBF this input.

server.go

@@ -1073,6 +1073,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		Signer:               cc.Wallet.Cfg.Signer,
 		Wallet:               newSweeperWallet(cc.Wallet),
 		TickerDuration:       cfg.Sweeper.BatchWindowDuration,
+		Mempool:              cc.MempoolNotifier,
 		Notifier:             cc.ChainNotifier,
 		Store:                sweeperStore,
 		MaxInputsPerTx:       sweep.DefaultMaxInputsPerTx,