THRIFT-10. java: Descriptors used during serialization should be immutable objects

THRIFT-10. java: Descriptors used during serialization should be immutable objects

-Descriptor classes all have final members, making them immutable.
-Generated structs now have static constant versions of their TStruct and TField descriptors, and will be used during writing.
-Protocols that can benefit use static constants for various common returned descriptors.
-A duplicate FieldMetaData.java that should have been removed previously was also removed.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@741984 13f79535-47bb-0310-9956-ffa450edef68