Clean up after Atomic<T> conversions:

- change looping initialization of members to single-line array initialization, e.g.

for (int j = 0; j < X; j++) {
  ::new (&_region_mark_stats[i]) G1RegionMarkStats();
}

to

::new (&_region_mark_stats) G1RegionMarkStats[X]();

- resets should probably also use above construct instead of the explicit loops

- remove now unnecessary default constructors that were added because previously Atomic<T> required an explicit constructor

- ReferenceProcessorPhaseTimes::_ref_dropped should get an explicit initialzation in the initalizer list

Splt as necessary