Check attachment for LogContext.

@jamezp, @dmlloyd : btw, how do you expect the https://github.com/wildfly/wildfly/blob/master/logging/src/main/java/org/jboss/as/logging/LoggingDeploymentUnitProcessor.java#L97 to work?

Since, afaik, ClassLoaderLogContextSelector uses caller stack to match the classloader.
And since we register app's classloader against LogContextSelector, whereas cleanup is done in subsystem,
there is no way this will ever match -- hence leaking classloaders.

@ctomc ^^