ut-merge_sort: Add missing parameter to relative_memory_usage ctor.
serialization_sort: Merge evacuate_before_merging and evacuate_before_reporting into single evacuate method.
serialization_sort: Add set_available_memory(m1, m2, m3) method to match merge_sorter.
serialization_sort: Add evacuated_memory_usage() method to match merge_sorter.
serialization_sort: Reset file handler as soon as the last item is pulled out of the final merge phase.
Refactor ut-merge_sort, adding test/unit/merge_sort.h and ut-serialization_sort.