Thread created by NamedThreadFactory should be a daemon

[z1c0]

In src/main/java/org/java_websocket/util/NamedThreadFactory.java, when a new Thread gets created, it should be marked as a background thread.

thread.setDaemon(true);

I'm using this library in a context where I have no control over shutdown (I get no callback or notification) and this thread is preventing proper shutdown.

[PhilipRoman]

Looks like we already have open PR #1201 for this. I will review it when I have time and most likely merge.

[z1c0]

Great news! Thanks