Lightly serve stability

[japrescott]

try to stabilize lightly-serve by

• ensuring we correctly shutdown the server
• use ThreadingHTTPServer instead of HTTPServer

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 81.84%. Comparing base (8fc3a2e) to head (bc3efd1).

Files	Patch %	Lines
lightly/cli/serve_cli.py	0.00%	3 Missing :warning:
lightly/api/serve.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1550 +/- ## ========================================== - Coverage 81.89% 81.84% -0.05% ========================================== Files 144 144 Lines 6119 6115 -4 ========================================== - Hits 5011 5005 -6 - Misses 1108 1110 +2 ```

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