The change I made is to check GL_QUERY_RESULT_AVAILABLE before retrieving the query, to ensure that there won't be a stall. If the query result is not available yet, we'll leave it alone and try again in the next frame.
Had to make a few more changes than I'd liked, mostly because the TestResult mechanism wasn't designed for holding on to query objects for more than one frame. As well, I'm thinking that RetrieveQueriesCallback and ClearQueriesCallback could be merged together, if we wanted to go for more refactoring. For though now my strategy is to make as little changes as possible. Let me know what you think of the patch."