[Avro] Issue on OSGi due to static cache

Avro holds a cache of classloaders and other reflection related classes in a static map here:

https://github.com/apache/avro/blob/master/lang/java/avro/src/main/java/org/apache/avro/specific/SpecificData.java#L52-L57

Keys of that map are {{String}}s with classname.

The cache is never updated, and this breaks OSGi ability to update a new class definition, since avro will keep pointing to the old version.

This will lead to a situation where, on an OSGi platform, you are exposed to ClassCastException since the rest of the platform might see a different varsion than the old one cached by avro.

The attached fix is a simple way to entirely bypass this caching behavior with just public subclasses that turn off just the caching behavior.