Optimize class loading - make backward compatible with legacy behaviour

Now the defaults are:
+ org.aspectj.apache.bcel.useSingleRepositoryInstance (default: false)
+ org.aspectj.apache.bcel.useUnavailableClassesCache (default: false)
+ org.aspectj.apache.bcel.ignoreCacheClearRequests (default: false)

I.e. the new caching optimisations are opt-in instead of opt-out as
originally designed. This might change in the future, but for now
without any additional tests and experience with the new feature let us
be conservative and make the build green first.

I also added a few more code review findings concerning backward
compatibility, which was less than 100% even with all three flags
deactivated.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

bcel-builder/src/main/java/org/aspectj/apache/bcel/util/ClassLoaderRepository.java

 
 	//Cache not found classes as well to prevent unnecessary file I/O operations
 	public static final boolean useUnavailableClassesCache =
-		System.getProperty("org.aspectj.apache.bcel.useUnavailableClassesCache", "true").equalsIgnoreCase("true");
+		System.getProperty("org.aspectj.apache.bcel.useUnavailableClassesCache", "false").equalsIgnoreCase("true");
 	//Ignore cache clear requests to not build up the cache over and over again
 	public static final boolean ignoreCacheClearRequests =
-		System.getProperty("org.aspectj.apache.bcel.ignoreCacheClearRequests", "true").equalsIgnoreCase("true");
+		System.getProperty("org.aspectj.apache.bcel.ignoreCacheClearRequests", "false").equalsIgnoreCase("true");
 
 	//Second cache for the unavailable classes
 	private static Set<String> unavailableClasses = new HashSet<String>();
 			classesLoadedCount++;
 			return clazz;
 		} catch (IOException e) {
-			unavailableClasses.add(className);
+			if (useUnavailableClassesCache) {
+				unavailableClasses.add(className);
+			}
 			throw new ClassNotFoundException(e.toString());
 		}
 	}

weaver/src/main/java/org/aspectj/weaver/reflect/Java15AnnotationFinder.java

 
 	//Use single instance of Repository and ClassLoader
 	public static final boolean useSingleInstances =
-		System.getProperty("org.aspectj.apache.bcel.useSingleRepositoryInstance", "true").equalsIgnoreCase("true");
+		System.getProperty("org.aspectj.apache.bcel.useSingleRepositoryInstance", "false").equalsIgnoreCase("true");
 
 	static {
 		try {
 			if (useSingleInstances && staticBcelRepository == null)
 				staticBcelRepository = new ClassLoaderRepository(getClassLoader());
 			else
-				this.bcelRepository = new ClassLoaderRepository(getClassLoader());
+				this.bcelRepository = new ClassLoaderRepository(classLoaderRef);
 		}
 		else {
 			if (useSingleInstances && staticBcelRepository == null)
 				staticBcelRepository = new NonCachingClassLoaderRepository(getClassLoader());
 			else
-				this.bcelRepository = new NonCachingClassLoaderRepository(getClassLoader());
+				this.bcelRepository = new NonCachingClassLoaderRepository(classLoaderRef);
 		}
 	}