Balance the compaction phase for G1 Full GC

The compaction phase is currently not well balanced. The work done in this phase is determined by what regions the worker prepare in phase 2 (the prepare for compaction phase). Since there is no easy obvious way to steal work without adding a lot of synchronization this has to be solved in a smarter way.

There have been some discussions about how to create smaller work items that all can be done in parallel and than let the workers claim such work items. There might be other ways to tackle this as well.