Conflicted state set is a set of events (#1120)

In #1042 I incorrectly wrote that the conflicted state set is a set of
pairs (K, V). We later take the union of the conflicted state set and
the auth difference. The latter is a set of events (V) only.

Fix this by making the conflicted state set a set of events rather than
a set of pairs. That is, the conflicted state set is a a `Set[Event]`
instead of a `Set[((type, state key), event)]`.

changelogs/room_versions/newsfragments/1120.clarification

@@ -0,0 +1 @@
+Improve readability and understanding of the state resolution algorithms.