htlcswitch: add final htlc event to notifier

lnrpc/routerrpc/router.proto

@@ -619,6 +619,7 @@ message HtlcEvent {
         SettleEvent settle_event = 9;
         LinkFailEvent link_fail_event = 10;
         SubscribedEvent subscribed_event = 11;
+        FinalHtlcEvent final_htlc_event = 12;
     }
 }
 
@@ -649,6 +650,11 @@ message SettleEvent {
     bytes preimage = 1;
 }
 
+message FinalHtlcEvent {
+    bool settled = 1;
+    bool offchain = 2;
+}
+
 message SubscribedEvent {
 }