Ticket 48922 - Fix crash when deleting backend while import is running
Bug Description: If you delete a backend from the config while an
import is running the server can crash.
Fix Description: When deleting a backend from the config wait for the
backend instance to not be busy before removing the
indexes. Otherwise the dbenv is not stable and this
can cause the crash.
https://fedorahosted.org/389/ticket/48922
Reviewed by: nhosoi(Thanks!)
(cherry picked from commit 6c77c37ae5ce847ffa2bd75287dbd157c2f2d6af)