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.

routing/localchans/manager_test.go

@@ -46,7 +46,7 @@ func TestManager(t *testing.T) {
 
 	currentPolicy := channeldb.ChannelEdgePolicy{
 		MinHTLC:      minHTLC,
-		MessageFlags: lnwire.ChanUpdateOptionMaxHtlc,
+		MessageFlags: lnwire.ChanUpdateRequiredMaxHtlc,
 	}
 
 	updateForwardingPolicies := func(