Cause: Deleting backend code ldbm_instance_delete_instance_entry_callback had no checking for the ordinary operations accessing the backend instance.
Consequence: Even if some operations are still in progress,
the backend instance could be deleted and it crashes the server.
Fix: Added a reference counter to the backend and if the backend is in use, delete returns 0.
Result: Backend deletion fails if the backend is still being used and it does not crash the server.