Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=54507
Don't start expiring sessions until the web application is fully started. Stop expiring sessions as soon as the web application is stopped.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1441409 13f79535-47bb-0310-9956-ffa450edef68