Refactor walk

[justin-tay]

Closes #441, closes #467, closes #841, closes #868, closes #882

This contains breaking changes to those using the walk functionality

When using the walker with defaults the default across a $ref are properly resolved and used.

The behavior for the property listener is now more consistent whether or not validation is enabled. Previously if validation is enabled but the property is null the property listener is not called while if validation is not enabled it will be called. Now the property listener will be called in both scenarios.

The following are the breaking changes to those using the walk functionality. Previously there wasn't a way to get the JsonSchema of the schema that was referenced by the schemaNode and it was unclear what the schemaLocation or evaluationPath was referring to.

WalkEvent	Field	Change	Notes
schemaLocation	Removed	For keywords: getValidator().getSchemaLocation(). For items and properties: getSchema().getSchemaLocation()
evaluationPath	Removed	For keywords: getValidator().getEvaluationPath(). For items and properties: getSchema().getEvaluationPath()
schemaNode	Removed	getSchema().getSchemaNode()
parentSchema	Removed	getSchema().getParentSchema()
schema	New	For keywords this is the parent schema of the validator. For items and properties this is the item or property schema being evaluated.
node	Renamed	instanceNode
currentJsonSchemaFactory	Removed	getSchema().getValidationContext().getJsonSchemaFactory()
validator	New	The validator indicated by the keyword.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 62.03704% with 41 lines in your changes are missing coverage. Please review.

Project coverage is 78.76%. Comparing base (95911ba) to head (65be26c). Report is 1 commits behind head on master.

Files	Patch %	Lines
...ain/java/com/networknt/schema/utils/JsonNodes.java	52.17%	9 Missing and 2 partials :warning:
...ava/com/networknt/schema/ItemsValidator202012.java	0.00%	7 Missing :warning:
...ava/com/networknt/schema/PrefixItemsValidator.java	0.00%	7 Missing :warning:
...ava/com/networknt/schema/utils/JsonSchemaRefs.java	50.00%	3 Missing and 2 partials :warning:
...main/java/com/networknt/schema/ItemsValidator.java	60.00%	3 Missing and 1 partial :warning:
...main/java/com/networknt/schema/walk/WalkEvent.java	77.77%	2 Missing :warning:
src/main/java/com/networknt/schema/JsonSchema.java	92.30%	0 Missing and 1 partial :warning:
...a/com/networknt/schema/MaxPropertiesValidator.java	0.00%	1 Missing :warning:
...a/com/networknt/schema/MinPropertiesValidator.java	0.00%	1 Missing :warning:
...java/com/networknt/schema/PropertiesValidator.java	93.75%	0 Missing and 1 partial :warning:
... and 1 more

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #986 +/- ## ============================================ + Coverage 78.51% 78.76% +0.24% - Complexity 1866 1919 +53 ============================================ Files 169 172 +3 Lines 6088 6197 +109 Branches 1153 1199 +46 ============================================ + Hits 4780 4881 +101 + Misses 871 861 -10 - Partials 437 455 +18 ```

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