PSC sync: do not schedule hi events for lo participants (#3998, #4010)

Prevents the sync from attempting to schedule lo segments for hi participants and vice versa. Differs from the existing behavior in that, e.g., the presence of a (bad, hi-only) 6M event in the event history of a lo participant will not result in the lo participant being scheduled for the full course of hi events.

Extra bonus issue addressed: #4010. Eliminates "errors" about unsyncable informed consent events by not trying to sync them. These are known to be unsyncable and it is not likely that that will ever change. The logs don't need to be full of "errors" reminding us, especially when those logs may obscure actual errors logged due to a lo person having a hi event.