poh: fix microblock mis-ordering bug

[mmcgee-jump]

I have bad news. I was thinking about it, and I think our new pack->bank interaction is not correct. What if a bank tile gets context switched out after telling pack it's done but before sending the microblock to PoH, pack schedules another transaction involving that account to a different bank, and that microblock makes it to PoH before the first one does? They'll get stamped in a different order from how they were executed, right?