multi: rename due to required maxHTLC bit

We rename `ChanUpdateOptionMaxHtlc` to `ChanUpdateRequiredMaxHtlc`
as with the latest changes it is now required.

Similarly, rename `validateOptionalFields` to
`ValidateChannelUpdateFields`, export it to use it in a later commit.

lnwire/message_test.go

@@ -690,7 +690,7 @@ func newMsgChannelUpdate(t testing.TB, r *rand.Rand) *lnwire.ChannelUpdate {
 	// as being part of the ChannelUpdate, to pass
 	// serialization tests, as it will be ignored if the bit
 	// is not set.
-	if msgFlags&lnwire.ChanUpdateOptionMaxHtlc == 0 {
+	if msgFlags&lnwire.ChanUpdateRequiredMaxHtlc == 0 {
 		maxHtlc = 0
 	}