Ticket 47766 - Tombstone purging can crash the server if the backend is stopped/disabled
    
    Bug Description:  The tombstone purge thread issues a callback search that will start
                      reading id2entry, even if the backend is stopped.  This can crash the
                      server.
    
    Fix Description:  When performing a search and returning entries, check if the backend
                      is started before reading id2entry.
    
    https://fedorahosted.org/389/ticket/47766
    
    Reviewed by: nhosoi(Thanks!)
    (cherry picked from commit 559d70e0a1e3df75a66e41fd1c009b25ca6f6a08)