The code around incremental collection set building is very old, and due to changes like JDK-8343782 changes were done in places that do not really fit

In detail
- move implementations of incremental collection set building helpers out of header file
- _selected_group_cur_length is unnecessary and can be replaced by an (inexpensive) call
- some members about incremental building updated outside of the start/continue/stop_incremental_build() method
- assertions around G1CSetBuildType are out of date

Cleanup.