Optimize class loading

Optimize class loading

In our project we found out that during the build up of the spring context
the class loading takes a very long time.
Root cause is the huge amount of file I/O during pointcut class loading.
We are taking about ~250k file loads.

With these changes we managed to cut down the starting time by around 50%.

What we found out is that IMHO - the clear method of the ClassLoaderRepository
is called far too often -> in our settings this resulted in not a single cache
hit as the cache got cleared permanently.
Therefore we de-actived the cache clear calls inside the ClassLoaderRepository.

Secondly we changed the Java15AnnotationFinder in a way to not always create
new objects for the ClassLoaderRepository but re-use one static instance.
Otherwise we experienced >100k objects being created.

Last but not least we introduced a cache for unavailable classes so that
they do not have to be looked up using file I/O over and over again.

The whole behavior is configurable via
+ org.aspectj.apache.bcel.useSingleRepositoryInstance (default: true)
+ org.aspectj.apache.bcel.useUnavailableClassesCache (default: true)
+ org.aspectj.apache.bcel.ignoreCacheClearRequests (default: true)

Signed-off-by: Stefan Starke <stefan@starkeweb.org>
Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>