bugfix: enable on-demand loading/unloading with MultiClassLoader

- enforce loading of library in loadLibrary(), otherwise we cannot know
- don't unload libraries in destructor when on-demand-unloading is enabled

Author: rhaschke

include/class_loader/multi_library_class_loader.h

@@ -73,6 +73,8 @@ class MultiLibraryClassLoader
       for(unsigned int c = 0; c < active_loaders.size(); c++)
       {
         ClassLoader* current = active_loaders.at(c);
+        if (!current->isLibraryLoaded())
+          current->loadLibrary();
         if(current->isClassAvailable<Base>(class_name))
           return(current->createInstance<Base>(class_name));
       }
@@ -113,6 +115,8 @@ class MultiLibraryClassLoader
       for(unsigned int c = 0; c < active_loaders.size(); c++)
       {
         ClassLoader* current = active_loaders.at(c);
+        if (!current->isLibraryLoaded())
+          current->loadLibrary();
         if(current->isClassAvailable<Base>(class_name))
           return(current->createUnmanagedInstance<Base>(class_name));
       }

src/multi_library_class_loader.cpp

@@ -39,7 +39,10 @@ enable_ondemand_loadunload_(enable_ondemand_loadunload)
 
 MultiLibraryClassLoader::~MultiLibraryClassLoader()
 {
-  shutdownAllClassLoaders();
+  if (!isOnDemandLoadUnloadEnabled())
+    shutdownAllClassLoaders(); // don't unload libs to avoid SEVERE WARNING
+  // TODO: free ClassLoaders in active_class_loaders_
+  // However, we still need them in on-demand-load-unload mode. Otherwise we risk seg-faults.
 }
 
 std::vector<std::string> MultiLibraryClassLoader::getRegisteredLibraries()