lncfg: add new legacy protocol option to turn off gossip throttling

lncfg/protocol_legacy_off.go

@@ -20,3 +20,8 @@ func (l *LegacyProtocol) LegacyOnion() bool {
 func (l *LegacyProtocol) NoStaticRemoteKey() bool {
 	return false
 }
+
+// NoGossipThrottle returns true if gossip updates shouldn't be throttled.
+func (l *LegacyProtocol) NoGossipThrottle() bool {
+	return false
+}