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!)