Ticket #47602 - Make ldbm_back_seq independently support transactions
    
    Description: If ldbm_back_seq is called as a child of transaction, it
    fails to access the on-going transaction data.  This patch picks up
    the parent transaction if any, and it calls dblayer_read_txn_begin with
    the parent transaction.  If the read transaction is aborted by DEADLOCK,
    it retries.
    
    https://fedorahosted.org/389/ticket/47602
    
    Reviewed by mreynolds@redhat.com (Thank you, Mark!!)