Currently, for simplicity, G1 only tracks to-cset cards for to-young gen references. However, since during mixed gc we guarantee to evacuate them anyway, these could be tracked as to-cset cards too.

This affects both rebuilding and updating the remembered sets.

Also the remembered sets of the next collection set group(s) could be dumped on the card table already to not do that during gc.

May be extended to any collection set candidate that we are sure to evacuate in the next gc