add static and final modifiers before ThreadLocal property md5Holder

According to the Java doc, the ThreadLocal property should always be static(refer to http://docs.oracle.com/javase/7/docs/api/java/lang/ThreadLocal.html). Despite that properties in interface are static, but i prefer to explicitly declare them as static and final.
So I rename md5Holder to MD5_HOLDER and add the static and final modifiers before it. 
Would you agree?