The CombinedCachingServlet can become a bottleneck under heavy load

The way synchonization is implemented in the CombindedCachingServlet can hang the system under heavy load, this happened with jboss.jira.com. The JBoss people have submitted a reasonable looking patch attached to https://support.atlassian.com/browse/JSP-6090. We need to incorperate this change into the atlassian-cache-servlet project and include that new version in JIRA. Confluence should also get this change.