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.

sweep/interface.go

@@ -9,7 +9,7 @@ import (
 type Wallet interface {
 	// PublishTransaction performs cursory validation (dust checks, etc) and
 	// broadcasts the passed transaction to the Bitcoin network.
-	PublishTransaction(tx *wire.MsgTx) error
+	PublishTransaction(tx *wire.MsgTx, label string) error
 
 	// ListUnspentWitness returns all unspent outputs which are version 0
 	// witness programs. The 'minconfirms' and 'maxconfirms' parameters