dns: make var to const if possible

[XadillaX]

Refs: #13732

Checklist

• make -j4 test
• commit message follows commit guidelines

Affected core subsystem(s)

dns

[XadillaX]

How about var to let?

[XadillaX]

duplicate with #13756

[gibfahn]

How about var to let?

I think the gating factor is whether there will be deoptimisations in V8. If you check and confirm that there won't be then I don't see the harm in raising a PR (and I'd be +1 on it). If we move away from var entirely we can use the no-var and prefer-const eslint rules.