Reduce usage of tf.compat.v1 APIs

TensorBoard have migrated to TF 2.0 APIs but there are some usages of V1 APIs due to not-so-compatible changes. Since we are interested in moving off of tf.compat.v1, we will either have to find a replacement or introduce some changes in behavior.

Do note that this list exclude v1 API usages in tests (they are mainly written in graph style and is tracked in #1705) and TB v1 summary.

• #1725: tf.compat.v1.app -> absl.app
• #1726: tf.compat.v1.flags -> absl.flags
• #1733: tf.compat.v1.gfile.GFile -> io.gfile.GFile
• #1733: tf.compat.v1.gfile.Open -> io.gfile.GFile
• #1734: tf.compat.v1.image.resize_images -> tf.image.resize
• #1734: tf.compat.v1.image.resize_nearest_neighbor -> tf.image.resize
• #1730: tf.compat.v1.GraphDef -> TB proto
• #1730: tf.compat.v1.MetaGraphDef -> TB proto
• #1721: tf.compat.v1.RunMetadata -> TB proto
• #1720: tf.compat.v1.SessionLog -> TB proto
• #2096: tf.compat.v1.pywrap_tensorflow.NewCheckpointReader -> tf.train.load_checkpoint
• #2157: tf.compat.v1.checkpoint_exists -> tf.io.gfile.glob() for the prefix
• tf.compat.v1.Session -> Use eager mode
• tf.contrib.ffmpeg -> tf.audio.encode_wav or python stdlib's wave module
• tf.contrib.summary.create_db_writer -> replace with python implementation (obsolete)
• tf.compat.v1.pywrap_tensorflow.EventsWriter -> migrated to tensorflow.python._pywrap_events_writer private API via #2096 and then #2683.
• tf.compat.v1.io.tf_record_iterator -> tracked in #1711
• tf.compat.v1.train.summary_iterator - should become our own wrapper of tf_record_iterator for the places we need it (only tests)
• #3185: tf.compat.v1.pywrap_tensorflow.PyRecordReader_New -> converted to be only a legacy fallback for when we can't use tf.compat.v1.io.tf_record_iterator

Review the remaining unchecked checklist items, especially the tf.compat.v1.Session migration and the summary_iterator work, and read the referenced issue #1711. Trace the current TensorFlow compatibility usage and related tests before deciding the replacement; done means the remaining listed APIs are migrated without breaking the stated test exclusions.