Ticket 47771 - Performing deletes during tombstone purging results in operation errors
    
    Bug Description:  An operations error can occur when deleting entry while
                      tombstone purging is happening.  The error occurs when it
                      tries the lock the parent entry, but the parent entry was
                      replaced in the cache before it could be locked.
    
    Fix Description:  Return a special error code when cache_lock_entry fails because
                      the entry was marked as deleted.  Then try to grab the entry
                      again and lock it.
    
    https://fedorahosted.org/389/ticket/47771
    
    Reviewed by: rmeggins & nhosoi(Thanks!!)