For some optimizations for the Rebuilding remembered set concurrently feature (JDK-8180415) eager reclaim needs to provide information about the object selection. Rebuilding remembered sets also needs to do some special work when eagerly rebuilding - improve the existing code to add a callback to G1ConcurrentMark to not pollute the eager reclaim code that much

- i.e. whether an object will ever be a potential humongous reclaim candidate
- add callback for concurrent mark related work