dnsdist: Move constants to proper namespace #7678
Redo of #5669. Separate PR because the rebase involved some manual work (some code got removed, moved around, etc. since), and this way we can compare the two PRs to make sure I did not miss anything.
I have now run the old tests under the new namespaces, and as long as I tolerate the deprecation warnings during startup in the test suite, all tests pass.
Ready for review, merge.