changes from https://github.com/wizyoung/YOLOv3_TensorFlow

by Seongju Kim, kareus1@khu.ac.kr

1] changed TextLineDataset to TFRecordDataset. (also changed data parsing in data utils and eval utils)
2] fixed restore-does-not-exist problem in train/eval mode
3] fixed saver to save the parameter only when save-optimizer option is true
4] changed parameter 'mode' to bool value 'is_training' in data util functions (string value 'mode' is passed as byte string, so functions do not evaluate if-clauses as expected. ex) 'train' != b'train')
5] wrote TFRecord binary iterator, which runs without tf session (references: https://github.com/pgmmpk/tfrecord )