Reduce memory usage and improve performance

[justin-tay]

This reduces memory usage and improves performance in particular when a lot of assertions are generated, typically with a large input data set and with the use of oneOf. In such cases the application may hang as it attempts to repeatedly aggregate the assertions into the LinkedHashSet.

[codecov-commenter]

Codecov Report

Attention: 30 lines in your changes are missing coverage. Please review.

Comparison is base (c768bc1) 78.78% compared to head (5b30462) 78.63%.

Files	Patch %	Lines
.../main/java/com/networknt/schema/utils/SetView.java	76.92%	8 Missing and 10 partials ⚠️
...va/com/networknt/schema/DependenciesValidator.java	80.00%	0 Missing and 2 partials ⚠️
...in/java/com/networknt/schema/DependentSchemas.java	71.42%	0 Missing and 2 partials ⚠️
...a/com/networknt/schema/PropertyNamesValidator.java	60.00%	0 Missing and 2 partials ⚠️
...main/java/com/networknt/schema/AllOfValidator.java	83.33%	0 Missing and 1 partial ⚠️
...main/java/com/networknt/schema/AnyOfValidator.java	94.73%	0 Missing and 1 partial ⚠️
...ava/com/networknt/schema/ItemsValidator202012.java	80.00%	0 Missing and 1 partial ⚠️
...main/java/com/networknt/schema/OneOfValidator.java	90.90%	0 Missing and 1 partial ⚠️
...ava/com/networknt/schema/PrefixItemsValidator.java	80.00%	0 Missing and 1 partial ⚠️
...java/com/networknt/schema/PropertiesValidator.java	92.85%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #966      +/-   ##
============================================
- Coverage     78.78%   78.63%   -0.16%     
- Complexity     1768     1809      +41     
============================================
  Files           160      161       +1     
  Lines          5798     5893      +95     
  Branches       1081     1116      +35     
============================================
+ Hits           4568     4634      +66     
- Misses          825      833       +8     
- Partials        405      426      +21     

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