cli: note when data server is used implicitly

[wchargin]

Summary:
We’d like to set --load_fast=auto as the default for TensorBoard 2.5.
To make that less surprising, we now print an informational message when
--load_fast is set to auto and the data server is actually used. We
don’t show it with --load_fast=true; if you pass that, we assume that
you know what you’re doing. The message looks like:

$ tensorboard --logdir /tmp/logs --bind_all --load_fast=auto
2021-03-17 11:41:51.151546: W tensorflow/stream_executor/platform/default/dso_loader.cc:60] Could not load dynamic library 'libcudart.so.11.0'; dlerror: libcudart.so.11.0: cannot open shared object file: No such file or directory
2021-03-17 11:41:51.151567: I tensorflow/stream_executor/cuda/cudart_stub.cc:29] Ignore above cudart dlerror if you do not have a GPU set up on your machine.

NOTE: Using experimental fast data loading logic. To disable, pass
    "--load_fast=false" and report issues on GitHub. More details:
    https://github.com/tensorflow/tensorboard/issues/4784

TensorBoard 2.5.0a0 at http://localhost:6007/ (Press CTRL+C to quit)

Test Plan:
Run with --load_fast set to false, auto, and true, and note that
the message only appears when set to auto. Then uninstall the data
server and run with auto, and note that the message does not appear.

wchargin-branch: cli-data-server-message