multi: update WalletController PublishTransaction to include label

Add label parameter to PublishTransaction in WalletController
interface. A labels package is added to store generic labels that are
used for the different types of transactions that are published by lnd.

To keep commit size down, the two endpoints that require a label
parameter be passed down have a todo added, which will be removed in
subsequent commits.

lnwallet/interface.go

@@ -224,8 +224,9 @@ type WalletController interface {
 	// already known transaction, ErrDoubleSpend is returned. If the
 	// transaction is already known (published already), no error will be
 	// returned. Other error returned depends on the currently active chain
-	// backend.
-	PublishTransaction(tx *wire.MsgTx) error
+	// backend. It takes an optional label which will save a label with the
+	// published transaction.
+	PublishTransaction(tx *wire.MsgTx, label string) error
 
 	// SubscribeTransactions returns a TransactionSubscription client which
 	// is capable of receiving async notifications as new transactions