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

@@ -1331,6 +1331,7 @@ public PluginWrapper whichPlugin(Class c) {
     /**
      * Orderly terminates all the plugins.
      */
+    @SuppressFBWarnings(value = "DM_GC", justification = "Garbage collection is required to release files held by plugin classloaders")
     public void stop() {
         for (PluginWrapper p : activePlugins) {
             p.stop();
@@ -1340,6 +1341,9 @@ 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();
     }
 
     /**