Implement liveness check for notebook extensions

Currently, each TensorBoard process writes its meta-information to a
file in the shared .tensorboard-info temp directory, and tries to
clean up the file on graceful exit. This has two problems on Windows:

• The base temporary directory %TMP% is never automatically cleaned,
  even after logout or reboot.
• It is not possible to gracefully shut down an arbitrary process
  given its PID.

The result is that most any TensorBoards started by %tensorboard will
leave their info files around forever, unless manually cleaned up, and
the instructions suggested to the user (“use !kill …”) are not
adequate to effect this cleanup. See #2481.

We can ameliorate this by implementing the liveness check mentioned in a
TODO in manager.py, cleaning up the dead info file on failure:

https://github.com/tensorflow/tensorboard/blob/c1c2771a5ed5ce1b9ad605dd1834616905c2f190/tensorboard/manager.py#L453-L458

We can also provide a notebook.kill(pid) function with implementation
something vaguely like

def kill(pid):
  if os.name == "nt":
    subprocess.check_output(["taskkill", "/pid", str(int(pid)), "/f"])
    manager.remove_info_file(pid)
  else:
    os.kill(pid, signal.SIGTERM)

and then replace the “reusing TensorBoard” user-facing messaging on
Windows with something like

template = (
    "Reusing TensorBoard on port {port} (pid {pid}), started {delta} ago. "
    "To kill it, run `from tensorboard import notebook; notebook.kill({pid})`."
)

though that incantation still is a bit of a mouthful.

Start with the liveness-check TODO in manager.py around the referenced lines, then inspect the notebook entry point and existing info-file handling. Trace how dead processes are detected and how Windows cleanup differs from other platforms. Done means dead info files are removed, notebook.kill(pid) handles the platform-specific shutdown, and the Windows reuse message points users to it.