routerrpc+htlcswitch: move intercepted htlc tracking to switch

In this commit we move the tracking of the outstanding intercepted htlcs
to InterceptableSwitch. This is a preparation for making the htlc
interceptor required.

Required interception involves tracking outstanding htlcs across
multiple grpc client sessions. The per-session routerrpc
forwardInterceptor object is therefore no longer the best place for
that.

server.go

@@ -1786,6 +1786,12 @@ func (s *server) Start() error {
 		}
 		cleanup = cleanup.add(s.htlcSwitch.Stop)
 
+		if err := s.interceptableSwitch.Start(); err != nil {
+			startErr = err
+			return
+		}
+		cleanup = cleanup.add(s.interceptableSwitch.Stop)
+
 		if err := s.chainArb.Start(); err != nil {
 			startErr = err
 			return