Configure worker for Undertow’s access log to use daemon threads

Previously, the worker used non-daemon threads which meant that they
prevented the JVM from shutting down. Ideally, we’d avoid this problem
by closing the worker and access log receiver as part of stopping
Undertow, however, due to an apparent bug in Undertow [1], it’s not
possible to do so cleanly.

This commit configures the access log worker to use daemon threads so
that they do not prevent the JVM from shutting down. Unfortunately,
this means that the threads will still be running after the context has
been closed but before the JVM shuts down but that appears to be
unavoidable due to the aforementioned Undertow bug.

Closes gh-4793

[1] https://issues.jboss.org/browse/UNDERTOW-597

spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowEmbeddedServletContainerFactory.java

@@ -405,8 +405,8 @@ public class UndertowEmbeddedServletContainerFactory
 
 	private XnioWorker createWorker() throws IOException {
 		Xnio xnio = Xnio.getInstance(Undertow.class.getClassLoader());
-		OptionMap.Builder builder = OptionMap.builder();
-		return xnio.createWorker(builder.getMap());
+		return xnio.createWorker(
+				OptionMap.builder().set(Options.THREAD_DAEMON, true).getMap());
 	}
 
 	private void registerServletContainerInitializerToDriveServletContextInitializers(