contractcourt+lnrpc+lnd: s/BreachArbiter/BreachArbitrator

Since we have two other examples of XArbitrator, we rename
BreachArbiter to BreachArbitrator to keep things consistent.
The aim is to reduce the amount of lore you need to know to
intuit where things are or what they do.

contractcourt/channel_arbitrator_test.go

@@ -1430,7 +1430,7 @@ func TestChannelArbitratorPersistence(t *testing.T) {
 // is breached by the remote node. In these cases we expect the
 // ChannelArbitrator to properly execute the breachResolver flow and then
 // gracefully exit once the breachResolver receives the signal from what would
-// normally be the breacharbiter.
+// normally be the BreachArbitrator.
 func TestChannelArbitratorForceCloseBreachedChannel(t *testing.T) {
 	log := &mockArbitratorLog{
 		state:     StateDefault,