Reset lastHandledEventId on speculative WFT

[Quinn-With-Two-Ns]

On a rejected speculative WFT we also need to reset lastHandledEventId so we don't accidentally skip new workflow tasks.

  public void handleEvent(HistoryEvent event, boolean hasNextEvent) {
    long eventId = event.getEventId();
    if (eventId <= lastHandledEventId) {
      // already handled
      return;
    }
...

We also should delete any new state machines created by the speculative task

closes #1880

[cretz]

I have no insight in the correctness/danger of this change and can only hope tests catch any issues. I will defer this review to others (if there are any others)