Fix logic for mutual conflict between two events

Fix logic for mutual conflict between two events

Prior to this commit, detecting whether two
events were in conflict involved looking at all
of the events with respect to all others. However,
you must instead *only* look at how the event `e`
and `e'` relate to the other event's history:
we wouldn't care if some other event is in conflict
with `e'` if `e` is not that event