data: check liveness before blessing data server

[wchargin]

Summary:
Issue #4844 shows that there are circumstances when communication
between TensorBoard and the local data server process cannot be
established. As of this patch, we send a trivial RPC to the data server
and wait for its response before committing to use the new loading
paths. This adds about 1–5ms total time to the happy path on my machine,
with a worst case penalty of 5s due to the timeout. If the server is not
reachable, we print a warning and fall back to the legacy paths.

Test Plan:

• Test with normal working TensorBoard.

• Simulate failed data server connection by changing the definition of
  addr to "localhost:%d" % (port + 777) on line 175. This should
  print the UNAVAILABLE/“failed to connect” message from [2.5] Failed to pick subchannel #4844.

• Simulate slow data server by adding the following to cli.rs:

  tokio::time::sleep(Duration::from_secs(3)).await;

  Add this right before the Server::builder().(...).await? call at
  the end of main: i.e., after we write the port file, but before we
  actually respond to requests. Note that TensorBoard still works with
  a 3-second delay, but that it actually delays printing the startup
  message for those 3 seconds as it determines which data provider to
  use.

• Simulate extra-slow data provider as above but waiting 6 seconds,
  and note that TensorBoard prints a DEADLINE_EXCEEDED error, falls
  back to the legacy paths, and shows valid data.

wchargin-branch: data-liveness-check

[wchargin]

If all looks good, I’d like to cherry-pick this into 2.5.0.

[stephanwlee]

Unusual merge: for sake of expediency for 2.5.0 release, I am merging it on behalf of wchargin.