Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56578
Correct regression in the fix for 56339 that prevented sessions from expiring when using clustering.
The effect of removing expiring = true; from the DeltaSession is that if parallel calls are made to expire() then there is a slightly greater chance of some of those calls blocking while the first completes. This is because expiring is now set to true a little later.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1600839 13f79535-47bb-0310-9956-ffa450edef68