fix(stdlib): use vector tokio runtime instead of using a worker thread

[esensar]

Summary

This removes the needless worker thread from dns_lookup and just blocks on the current tokio runtime (if available). Worker thread code was blocking anyways, this just removes all of that coordination code.

I am not sure if this will actually make much of an impact, but I wanted to simplify this, because we have seen some issue with dns_lookup and this whole worker thread thing made things more complicated unnecessarily, because it was still blocking anyways.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

Ran existing tests as well as running it inside vector. It seems to work as before.

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on
  our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Our CONTRIBUTING.md is a good starting place.
• If this PR introduces changes to LICENSE-3rdparty.csv, please
  run dd-rust-license-tool write and commit the changes. More details here.
• For new VRL functions, please also create a sibling PR in Vector to document the new function.

References

• Related: fix(stdlib): properly detect tokio runtime in dns_lookup #882

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