memory creep on deleteTicket

We have been seeing the following eviction problem when tickets are expired but the ehcache threshold has not been reached.  This can be a problem because old ticket references "hang around" in the cache despite being expired.

See http://lists.terracotta.org/pipermail/ehcache-list/2011-November/000423.html

By updating the ticket to null, ticket can garbage collected and prevent memory creep.