Increase the default maximum size of the executor used for async writes

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1589035 13f79535-47bb-0310-9956-ffa450edef68
WebSocket has own section in changelog.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1589039 13f79535-47bb-0310-9956-ffa450edef68
Refactor server container shutdown into the destroy method
Destroy the thread group on shutdown
Log a warning if the thread group can't be destroyed

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1589043 13f79535-47bb-0310-9956-ffa450edef68