FINERACT-2090: Refactor and cleanup Loan entity

[kjozsa]

Description

This PR is part of a larger changeset, covered by the ticket FINERACT-2090, aiming to seriously improve the Loan module's code quality. These changes focus solely on cleaning up the Loan class, which was ~7000 lines long at the develop HEAD originally.

All changes are either of the following kinds:

• introducing Lombok and removing unnecessary code parts of the Loan entity
• rewriting imperative filter/find methods using the stream APIs, hence improving readability and reducing potential bugs, trading in for a small performance cost
• fixing cosmetic and inconsistent formatting issues

Applying these changes helped to cut over 600 lines of code from this class alone, keeping the same functionality and seriously improving readability of the remaining parts.

Checklist

Please make sure these boxes are checked before submitting your pull request - thanks!

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• [ X] Create/update unit or integration tests for verifying the changes made.

• [ X] Follow coding conventions at https://cwiki.apache.org/confluence/display/FINERACT/Coding+Conventions.

• [ X] Add required Swagger annotation and update API documentation at fineract-provider/src/main/resources/static/legacy-docs/apiLive.htm with details of any API changes

• [ X] Submission is not a "code dump". (Large changes can be made "in repository" via a branch. Ask on the developer mailing list for guidance, if required.)

FYI our guidelines for code reviews are at https://cwiki.apache.org/confluence/display/FINERACT/Code+Review+Guide.

[adamsaghy]

LGTM