wpiutil: uv: Add LoopClosing status to Handle (#1647)

Useful for EventLoopRunner to know if a stop is requested, or close is happening for another reason.

wpiutil/src/main/native/cpp/EventLoopRunner.cpp

@@ -49,7 +49,10 @@ EventLoopRunner::~EventLoopRunner() { Stop(); }
 void EventLoopRunner::Stop() {
   ExecAsync([](uv::Loop& loop) {
     // close all handles; this will (eventually) stop the loop
-    loop.Walk([](uv::Handle& h) { h.Close(); });
+    loop.Walk([](uv::Handle& h) {
+      h.SetLoopClosing(true);
+      h.Close();
+    });
   });
   m_owner.Join();
 }