Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56905
Make destruction on web application stop of thread group used for WebSocket connections more robust.
Patch by kkolinko with minor tweaks

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1622251 13f79535-47bb-0310-9956-ffa450edef68