DependencyGraph: use ConfigObject*, not Object*

[Al2Klimov]

This saves dynamic_cast<ConfigObject*> + if() on every item of GetChildren().

TODO

• DependencyGraph: switch "parent" and "child" terminology #10263

[Al2Klimov]

• To be clear, this is not a requirement for Fix broken downtime comment sync #10000. But as you re-built the DG there and will re-work that due to DependencyGraph: switch "parent" and "child" terminology #10263 anyway, I think it makes sense to make the DG proper right away. Unless, of course, you don't like this PR.

[Al2Klimov]

• The point here is: Fix broken downtime comment sync #10000 as-is now traverses across plain Object*s and has to dynamic_cats. Not dramatic, but also not necessary. (Fix broken downtime comment sync #10000 (comment))

[yhabteab]

If this PR is just about replacing Object* with ConfigObject*, why did you make so many unrelated changes? For example, why did you replace std::map with std::unordered_map, knowing that this will be changed with #10000 anyway? If you want me to review this, then please drop all the unrelated changes that are not related to what the PR title advertises.

[Al2Klimov]

I've removed the top commit. (af064c8)