Trac Ticket #290 - server hangs during shutdown if betxn pre/post op fails
    
    https://fedorahosted.org/389/ticket/290
    
    Fix description: The logic to call dblayer_txn_abort in the
    ldbm_back update functions was wrong.  If the operations fail,
    the abort function has to be called if dblayer_txn_begin is
    already called and dblayer_txn_commit is not.  This patch
    checks the txn value set by dblayer_txn_commit to determine
    to call dblayer_txn_abort or not.