parallel: report progress for StreamTrees

This assigns an id to each tree root and then keeps track of how many
tree loads (i.e. trees referenced for the first time) are pending per
tree root. Once a tree root and its subtrees were fully processed there
are no more pending tree loads and the tree root is reported as
processed.

internal/checker/checker.go

@@ -376,7 +376,7 @@ func (c *Checker) Structure(ctx context.Context, errChan chan<- error) {
 		c.blobRefs.M.Insert(h)
 		c.blobRefs.Unlock()
 		return blobReferenced
-	})
+	}, nil)
 
 	defer close(errChan)
 	for i := 0; i < defaultParallelism; i++ {