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.

utxonursery.go

@@ -194,7 +194,7 @@ type NurseryConfig struct {
 
 	// PublishTransaction facilitates the process of broadcasting a signed
 	// transaction to the appropriate network.
-	PublishTransaction func(*wire.MsgTx) error
+	PublishTransaction func(*wire.MsgTx, string) error
 
 	// Store provides access to and modification of the persistent state
 	// maintained about the utxo nursery's incubating outputs.
@@ -867,7 +867,7 @@ func (u *utxoNursery) sweepCribOutput(classHeight uint32, baby *babyOutput) erro
 
 	// We'll now broadcast the HTLC transaction, then wait for it to be
 	// confirmed before transitioning it to kindergarten.
-	err := u.cfg.PublishTransaction(baby.timeoutTx)
+	err := u.cfg.PublishTransaction(baby.timeoutTx, "")
 	if err != nil && err != lnwallet.ErrDoubleSpend {
 		utxnLog.Errorf("Unable to broadcast baby tx: "+
 			"%v, %v", err, spew.Sdump(baby.timeoutTx))