watchtower+htlcswitch: update client tower logic to recognize safu commitments

In this commit, we update the tower+link logic to tag a commitment as
the new (tweakless) format if it applies. In order to do this, the
BackupTask method has gained an additional parameter to indicate the
type of commitment that we're attempting to upload. This new tweakless
bool is then threaded through all the way to back up task creation to
ensure that we make the proper input.Input.

Finally, we've added a new test case for each existing test case to test
each case w/ and w/o the tweakless modifier.

htlcswitch/interfaces.go

@@ -175,6 +175,8 @@ type TowerClient interface {
 	// state. If the method returns nil, the backup is guaranteed to be
 	// successful unless the tower is unavailable and client is force quit,
 	// or the justice transaction would create dust outputs when trying to
-	// abide by the negotiated policy.
-	BackupState(*lnwire.ChannelID, *lnwallet.BreachRetribution) error
+	// abide by the negotiated policy. If the channel we're trying to back
+	// up doesn't have a tweak for the remote party's output, then
+	// isTweakless should be true.
+	BackupState(*lnwire.ChannelID, *lnwallet.BreachRetribution, bool) error
 }