current implementation of MapWritable and AbstractMapWritable do not
track class usage. Class name is still serialized in write() to disk
even if no instance of such class exists in stored table anymore.
I propose to add usage tracking to AbstractMapWritable addToMap(Class
clazz) and provide function removeFromMap which will be called by
MapWritable. I can code such patch,. but i am inexperienced in hadoop
development, What hadoop branch should i target?