Spotbugs: SS_SHOULD_BE_STATIC

[Fokko]

This PR fixes the Spotbugs SS_SHOULD_BE_STATIC rule. This class contains an instance final field that is initialized to a compile-time static value. Consider making the field static.

Description

Describe the goal of this PR, what problem are you fixing. If there is a corresponding issue (referenced above), it's
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Fixed the bug ...

Renamed the class ...

Added a forbidden-apis entry ...

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  as when there are insufficient resources?
• Class organization and design (how the logic is split between classes, inheritance, composition, design patterns)
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---

This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths.
• added integration tests.
• been tested in a test Druid cluster.

Check the items by putting "x" in the brackets for the done things. Not all of these items apply to every PR. Remove the
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---

For reviewers: the key changed/added classes in this PR are MyFoo, OurBar, and TheirBaz.

(Add this section in big PRs to ease navigation in them for reviewers.)

[jihoonson]

@Fokko thanks. Would you please update the PR description to say what's really fixed in this PR?

[Fokko]

@jihoonson Sure, I've added some lines.

[asdf2014]

LGTM 👍

[Fokko]

Thanks!

[leventov]

This PR might have caused Travis failures: #8135