Fix leader election worker thread not running as daemon

[archy-rock3t-cloud]

What type of PR is this?

/kind bug

What this PR does / why we need it:

The leader election worker thread was created without daemon=True; the preceding threading.daemon = True only sets an unused attribute on the threading module. It is now created with daemon=True so it does not block interpreter shutdown.

Which issue(s) this PR fixes:

Fixes #2619

Does this PR introduce a user-facing change?

Start the leader election worker thread as a daemon so it does not block process shutdown.

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[yliaog]

/lgtm
/approve

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: archy-rock3t-cloud, yliaog

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