class usage counter in MapWritable

[Radim Kolar]

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?