Crowd's caching client libraries are not threadsafe

Original issue title: Concurrent Modification Exception when iterating the list of username returned by UserManager in the client

There is a user poller in Studio's Google Apps integration that creates new users when new users have been found from Google.

If another class is iterating the list of usernames returned by Crowd's UserManager at the same time, a ConcurrentModificationException is thrown.

Log and stacktrace is listed below. The first line shows a new user is being added in the SyncServiceThread-Thread-21 thread, which broke the iteration in the http-2990-4 thread.

2010-08-31 20:36:56,790 SyncServiceThread-Thread-21 INFO testadmin 1236x12x1 wu60v5 /secure/admin/XmlRestore.jspa jira.poller.impl.SyncJobImpl Creating new user: ' ('"&;: '"&:
2010-08-31 20:36:56,861 http-2990-4 ERROR testadmin 1236x12x1 wu60v5 /secure/admin/XmlRestore.jspa sal.core.upgrade.PluginUpgrader Upgrade failed: null
java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at com.atlassian.agmp.openid.users.mgmt.jira.upgrade.UT001SetOpenIdDomainAttributeForExistingGoogleUsers.doUpgrade(UT001SetOpenIdDomainAttributeForExistingGoogleUsers.java:61)