Ticket 47381 - nsslapd-db-transaction-batch-val turns to -1
    
    Bug Description:  If you set nsslapd-db-transaction-batch-val to zero, assuming
                      it was set to a number greater than zero, it gets changed to "-1".
                      Also, once it is set to zero, it is impossible to change the value
                      unless you edit dse.ldif manually.
    
    Fix Description:  First, do not transform a zero value to "-1".  Second, allow the
                      changing of the value regardless what the current value is.  If
                      batch transactions are disabled, log a mesage if we try to "enable"
                      it, stating that you need to restart the server.
    
                      Also needed to improve the synchronization if disabling batch txns,
                      as its possible to disable it while flushing is occurring.
    
                      Finally did a little code cleanup.
    
    https://fedorahosted.org/389/ticket/47381
    
    Reviewed by: Noriko(Thanks!)