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.

watchtower/config.go

@@ -72,7 +72,7 @@ type Config struct {
 	//
 	// TODO(conner): replace with lnwallet.WalletController interface to
 	// have stronger guarantees wrt. returned error types.
-	PublishTx func(*wire.MsgTx) error
+	PublishTx func(*wire.MsgTx, string) error
 
 	// ListenAddrs specifies the listening addresses of the tower.
 	ListenAddrs []net.Addr