switch to new merkledag walk functions

EnumerateChildrenAsync has been renamed to WalkParallel to reflect the fact
that:

1. It visits the root.
2. It's parallel, not async.

To mirror this change, EnumerateChildren has also been renamed to Walk and now
behaves the same (except that it's not parallel).

gc/gc.go

@@ -170,10 +170,8 @@ func Descendants(ctx context.Context, getLinks dag.GetLinks, set *cid.Set, roots
 	}
 
 	for _, c := range roots {
-		set.Add(c)
-
-		// EnumerateChildren recursively walks the dag and adds the keys to the given set
-		err := dag.EnumerateChildren(ctx, verifyGetLinks, c, set.Visit)
+		// Walk recursively walks the dag and adds the keys to the given set
+		err := dag.Walk(ctx, verifyGetLinks, c, set.Visit)
 
 		if err != nil {
 			err = verboseCidError(err)