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.

discovery/gossiper.go

@@ -1520,7 +1520,7 @@ func (d *AuthenticatedGossiper) retransmitStaleAnns(now time.Time) error {
 		if !edge.MessageFlags.HasMaxHtlc() {
 			// We'll make sure we support the new max_htlc field if
 			// not already present.
-			edge.MessageFlags |= lnwire.ChanUpdateOptionMaxHtlc
+			edge.MessageFlags |= lnwire.ChanUpdateRequiredMaxHtlc
 			edge.MaxHTLC = lnwire.NewMSatFromSatoshis(info.Capacity)
 
 			edgesToUpdate = append(edgesToUpdate, updateTuple{