Bug Description: Deleting backend code ldbm_instance_delete_instance_
entry_callback had no checking for the ordinary operations accessing
the backend instance. Even if some operations are still in progress,
the backend instance could be deleted and it crashes the server.
Fix Description: Backend struct ldbm_instance had a member inst_ref_
count, which was not used. This patch converts the type PRInt32 to
Slapi_Counter and increments it when the backend instance is in use.
The delete code checks the counter and if it is greater than 0, it