Fix a bug in TensorBoard where ctrl+c does not shut it down after it serves its first HTTP request.

The problem was that socketserver.ThreadingMixIn was improperly configured to run its threads as daemons (so that it did not do so).

See https://docs.python.org/2/library/socketserver.html for more details.

TESTED:
- passed opensource_build
- passed unit tests
Change: 133724322