web_profile_server is not properly shutting down in astropy.vo.samp

Update: This issue was moved to pyvo following https://github.com/astropy/pyvo/pull/729

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Once astropy/astropy#1907 is merged, we should look into the following issue:
- Start hub with `samp_hub`
- Go to http://astrojs.github.io/sampjs/examples/monitor.html and register the client
- Enter `yes` to the terminal prompt
- Unregister the client
- Control-C the hub

After the above steps, the hub hangs while stopping. I think this is due to the `_web_profile_server` which is still running and has not been properly shut down.