Add tf.contrib.checkpoint.CheckpointManager for deleting old checkpoints

Removes a bit of boilerplate from training loops (making a prefix from a directory).

Also clarifies the recovery of checkpoint lists (like tf.train.Saver.recover_last_checkpoints, but automatic and more thorough). Adds a couple fields to the CheckpointState proto to support this.

Should live in contrib until I make it work well with tf.keras.Model.save_weights. When used together, save_weights needs to number its checkpoints. (There's a TODO for this.)

PiperOrigin-RevId: 208566198