Ensure that threads created to support WebSocket clients are stopped when those clients no longer need them.
Note that while this happens automatically for WebSocket client calls made by web applications, stand-alone clients must call the Tomcat specific method WsWebSocketContainer.destroy().

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1589103 13f79535-47bb-0310-9956-ffa450edef68