Clean up code

[jkosternl]

• Simplify code, add final keyword, use diamond operator, correct English, use lambdas, remove .toString, use .isEmpty(), use .addAll, and more
• Thanks to IntelliJ code analyses feature

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 90.90909% with 9 lines in your changes missing coverage. Please review.

Project coverage is 78.14%. Comparing base (48ca3c2) to head (f1c64c9). Report is 69 commits behind head on master.

Files with missing lines	Patch %	Lines
.../java/com/networknt/schema/utils/JsonNodeUtil.java	50.00%	1 Missing and 1 partial :warning:
...java/com/networknt/schema/utils/StringChecker.java	0.00%	0 Missing and 2 partials :warning:
src/main/java/com/networknt/schema/JsonSchema.java	85.71%	0 Missing and 1 partial :warning:
...n/java/com/networknt/schema/JsonSchemaFactory.java	66.66%	1 Missing :warning:
...ava/com/networknt/schema/PrefixItemsValidator.java	0.00%	0 Missing and 1 partial :warning:
...om/networknt/schema/format/IriReferenceFormat.java	0.00%	0 Missing and 1 partial :warning:
.../main/java/com/networknt/schema/utils/RFC5892.java	94.44%	0 Missing and 1 partial :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1130 +/- ## ============================================ - Coverage 78.90% 78.14% -0.76% - Complexity 1965 2061 +96 ============================================ Files 172 201 +29 Lines 6352 6850 +498 Branches 1255 1252 -3 ============================================ + Hits 5012 5353 +341 - Misses 867 1030 +163 + Partials 473 467 -6 ```

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[stevehu]

Java 8 can compile the code without any issues. Thanks.