Force garbage collection after stopping plugins

Java doesn't have any direct means to release unreferenced classloaders,
so we force GC to do that so it becomes possible to delete plugin jar on Windows.

Author: slonopotamus

core/src/main/java/hudson/PluginManager.java

@@ -1340,6 +1340,9 @@ public void stop() {
         // Work around a bug in commons-logging.
         // See http://www.szegedi.org/articles/memleak.html
         LogFactory.release(uberClassLoader);
+        // Java doesn't have any direct means to release unreferenced classloaders,
+        // so we force GC to do that so it becomes possible to delete plugin jar on Windows.
+        System.gc();
     }
 
     /**