Reload the checkpoint reader when the latest checkpoint has changed.
Reload the checkpoint reader when the latest checkpoint has changed.

The embedding projector plugin caches the checkpoint reader, which is pointing to the latest checkpoint when TensorBoard was started. But after some time, the saver will remove that old checkpoint file (keeps only N latest checkpoints), while the reader is still pointing to it.

Also add more tests regarding the V1 and V2 checkpoint versions.
Change: 138416977

tensorflow/tensorboard,tensorflow/tensorboard,ioeric/tensorboard,shakedel/tensorboard,ioeric/tensorboard,shakedel/tensorboard,francoisluus/tensorboard-supervise,agrubb/tensorboard,shakedel/tensorboard,agrubb/tensorboard,agrubb/tensorboard,tensorflow/tensorboard,qiuminxu/tensorboard,francoisluus/tensorboard-supervise,francoisluus/tensorboard-supervise,francoisluus/tensorboard-supervise,shakedel/tensorboard,shakedel/tensorboard,qiuminxu/tensorboard,ioeric/tensorboard,qiuminxu/tensorboard,francoisluus/tensorboard-supervise,ioeric/tensorboard,qiuminxu/tensorboard,tensorflow/tensorboard,agrubb/tensorboard,agrubb/tensorboard,tensorflow/tensorboard,ioeric/tensorboard,ioeric/tensorboard,qiuminxu/tensorboard,tensorflow/tensorboard,qiuminxu/tensorboard,shakedel/tensorboard,tensorflow/tensorboard,francoisluus/tensorboard-supervise,agrubb/tensorboard