[tf.data] Add `num_parallel_reads` argument to `tf.data.TFRecordDataset`.

This provides a convenient way to use the `tf.contrib.data.parallel_interleave()` idiom for reading multiple TFRecord files in parallel. In addition, the `filenames` argument to the initializer can now be a `tf.data.Dataset` of strings, which makes it easier to use `TFRecordDataset` with `Dataset.list_files()`.

PiperOrigin-RevId: 187384812