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6c797587c7
dirty_memory_manager tracks lsa regions (memtables) under region_group:s, in order to be able to pick up the largest memtable as a candidate for flushing. Just as region_group:s contain regions, they can also contain other region_group:s in a nested structure. It also tracks the nested region_group that contains the largest region in a binomial heap. This latter facility is no longer used. It saw use when we had the system dirty_memory_manager nested under the user dirty_memory_manager, but that proved too complicated so it was undone. We still nest a virtual region_group under the real region_group, and in fact it is the virtual region_group that holds the memtables, but it is accessed directly to find the largest memtable (region_group::get_largest_region) and so all the mechanism that sorts region_group:s is bypassed. Start to dismantle this house of cards by removing the subgroup sorting. Since the hierarchy has exactly one parent and one child, it's clearly useless. This is seen by the fact that we can just remove everything related. We still need the _subgroups member to hold the virtual region_group; it's replaced by a vector. I verified that the non-intrusive vector is exception safe since push_back() happens at the very end; in any case this is early during setup where we aren't under memory pressure. A few tests that check the removed functionality are deleted. Closes #11515