Precise heap traversal API

[qinsoon]

https://github.com/mmtk/mmtk-core/pull/1174 introduced an API that allows heap traversal. The API will enumerate objects based on the VO bit at the time of enumeration. So if there is GC or allocation going on at the same time, we may see objects being reclaimed, or missing objects. We may want a version of the API that blocks allocation and GC so we can have a consistent heap snapshot.