Mark DestructorThread as a daemon

[jxtps]

Mark DestructorThread as a daemon so the JVM can exit.

Motivation

Why are you making this change?
To fix #25

Summary

What did you change?
I added sThread.setDaemon(true), which marks the destructor thread as a so called 'daemon' thread so it doesn't stop the JVM from exiting when all (other) user threads have terminated.

How does the code work?
It marks the thread as a daemon (a opposed to user) thread. The JVM keeps running until all user threads exit. This thread never exits.

Why did you choose this approach?
Because this is a textbook daemon thread.

Test Plan

How did you test this change?
See @dreiss code example in #25

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