Asynchronous heuristic notification mechanism

By default, if report_heuristics is false then the terminator of a transaction does not get information of any heuristic outcomes. However, it's possible that a sys admin or something/someone else may want to know about heuristics. The OTS 1.0 standard suggested that the CORBA Notification service could be used in these situations, but that was removed subsequently. We should add back the support for out-of-band notification of heuristics, though not mandating any specific implementation mechanism.