Fix conflict detection between configs + history

Fix conflict detection between configs + history

The computation of K-partial alternatives requires
that we can determine whether or not `[e] + C`
for some event `e` is a valid configuration. There
was a subtlety in the code prior to the this commit
that caused UDPOR to incorrectly compute whether
or not `[e] + C` was a valid configuration. Specifically,
it was not sufficient to look at the difference between
the event's history and the events in the configuration
and checking *dependency*. You have to check *conflicts*:
it's possible that two events are dependent in a configuration
but are related to each other causally (and thus would
not be in conflict with one another)