state-res: fix tiebreaking comparisons

`node` needs to be swapped with `parent` here, otherwise the
`power_level` and `age` in a `TieBreaker` inserted on the following line
are always equal, because both sides are the values for `node` rather
than a different value of `parent` on each iteration, and so the only
thing that could possibly be different is the `event_id` because that's
correctly set to `parent`. This is why I was observing tiebreaks always
being sorted by `event_id` and experiencing incorrectly resolved states.

crates/ruma-state-res/src/lib.rs

@@ -340,7 +340,7 @@ where
             // Only push on the heap once older events have been cleared
             out.remove(node.borrow());
             if out.is_empty() {
-                let (power_level, age) = key_fn(node.clone()).await?;
+                let (power_level, age) = key_fn(parent.clone()).await?;
                 heap.push(Reverse(TieBreaker {
                     inv_power_level: -power_level,
                     age,