Null out ref manager ref in cleanup

git-svn-id: http://svn.terracotta.org/svn/ehcache@7953 b9324663-ca0f-0410-8574-be9b3887307d
merge fix from trunk

git-svn-id: http://svn.terracotta.org/svn/ehcache@7954 b9324663-ca0f-0410-8574-be9b3887307d
merge fix from trunk

git-svn-id: http://svn.terracotta.org/svn/ehcache@7955 b9324663-ca0f-0410-8574-be9b3887307d