Optimize class loading - make backward compatible with legacy behaviour

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>