Refactor set to list in execution context

[justin-tay]

This refactors the return result from a set to a list.

Instead of returning the result for validation/walk, the errors are instead aggregated in a list in the execution context. Clients calling validate from JsonSchema will get a List instead of a `Set.

This should result in better performance when there are a lot of errors in the run as it's a single arraylist that aggregates results and gets resized, and as hashCode is no longer needed to be called on all the messages as it is no longer a set.

This also changes the version in the pom to be 2.0.0-SNAPSHOT to make it clear that this is a major breaking change.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 90.57239% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.94%. Comparing base (48ca3c2) to head (8b71dac).
⚠️ Report is 103 commits behind head on master.

Files with missing lines	Patch %	Lines
...in/java/com/networknt/schema/DependentSchemas.java	33.33%	3 Missing and 1 partial ⚠️
...c/main/java/com/networknt/schema/NotValidator.java	69.23%	3 Missing and 1 partial ⚠️
...in/java/com/networknt/schema/ExecutionContext.java	66.66%	2 Missing and 1 partial ⚠️
...java/com/networknt/schema/JsonSchemaException.java	66.66%	3 Missing ⚠️
...main/java/com/networknt/schema/OneOfValidator.java	87.50%	2 Missing and 1 partial ⚠️
...java/com/networknt/schema/DynamicRefValidator.java	33.33%	2 Missing ⚠️
...m/networknt/schema/FailFastAssertionException.java	60.00%	2 Missing ⚠️
src/main/java/com/networknt/schema/JsonSchema.java	85.71%	2 Missing ⚠️
...main/java/com/networknt/schema/AnyOfValidator.java	96.87%	0 Missing and 1 partial ⚠️
...main/java/com/networknt/schema/ItemsValidator.java	93.33%	0 Missing and 1 partial ⚠️
... and 3 more
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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1197      +/-   ##
============================================
- Coverage     78.90%   77.94%   -0.96%     
- Complexity     1965     2037      +72     
============================================
  Files           172      202      +30     
  Lines          6352     6943     +591     
  Branches       1255     1217      -38     
============================================
+ Hits           5012     5412     +400     
- Misses          867     1082     +215     
+ Partials        473      449      -24     

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