During full gc G1 abandons (drops) the entire collection set. It consists of
- G1CollectionSet (a set of G1CSetCandidateGroup)
- G1CollectionSetCandidates (a set of G1CSetCandidateGroup)
- and the young_gen_cset_group (also a G1CSetCandidateGroups) owned by G1CollectedHeap

During full gc we need to clear/drop them ("abandon") all of these. Currently the code for each of these components is located in different parts of the code.
Move them together.