Improved the readability and fixed few java warnings

[mrsrinivas]

Description

Fixed the readability and fixed few java warnings

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This PR has:

• been self-reviewed.
• 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 or updated version, license, or notice information in licenses.yaml
• 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.

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Key changed/added classes in this PR

• sql/src/main/java/org/apache/druid/sql/calcite/expression/builtin/TimeArithmeticOperatorConversion.java
• sql/src/main/java/org/apache/druid/sql/calcite/filtration/ConvertSelectorsToIns.java

[mrsrinivas]

retest

[gianm]

LGTM but there is a checkstyle issue reported. Could you please look into this:

[INFO] --- maven-checkstyle-plugin:3.0.0:checkstyle (default-cli) @ druid-sql ---

[INFO] Starting audit...

[ERROR] /home/travis/build/apache/druid/sql/src/main/java/org/apache/druid/sql/calcite/expression/Expressions.java:268: 'else' child has incorrect indentation level 8, expected level should be 6. [Indentation]

[mrsrinivas]

@gianm thank you for the review. I fixed it.

[gianm]

LGTM, thanks!