Refactor walk and update docs

[justin-tay]

Refactors some of the walk classes and also updates the documentation to reflect the changes for 2.x.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 78.72340% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.88%. Comparing base (48ca3c2) to head (9caad32).
⚠️ Report is 107 commits behind head on master.

Files with missing lines	Patch %	Lines
...main/java/com/networknt/schema/SchemaRegistry.java	16.66%	10 Missing ⚠️
...m/networknt/schema/walk/ApplyDefaultsStrategy.java	82.60%	2 Missing and 2 partials ⚠️
src/main/java/com/networknt/schema/Schema.java	40.00%	3 Missing ⚠️
...java/com/networknt/schema/utils/JsonNodeTypes.java	0.00%	1 Missing and 1 partial ⚠️
...main/java/com/networknt/schema/walk/WalkEvent.java	0.00%	1 Missing ⚠️
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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1202      +/-   ##
============================================
- Coverage     78.90%   77.88%   -1.02%     
+ Complexity     1965     1925      -40     
============================================
  Files           172      200      +28     
  Lines          6352     6769     +417     
  Branches       1255     1190      -65     
============================================
+ Hits           5012     5272     +260     
- Misses          867     1054     +187     
+ Partials        473      443      -30     

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