Description
HADOOP-16621 removed two public API methods:
1. o.a.h.security.token.Token(TokenProto tokenPB) --> replaced by o.a.h.security.ipc.ProtobufHelper.tokenFromProto()
2. o.a.h.security.token.Token.toTokenProto() --> replaced by o.a.h.security.ipc.ProtobufHelper.protoFromToken()
Protobuf is declared private. Should we make it public now?