Refactor to make more fields final

[justin-tay]

Refactor to make more fields final to ensure they aren't inadvertently changed and possibly improve performance marginally.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.38462% with 10 lines in your changes missing coverage. Please review.

Project coverage is 78.88%. Comparing base (48ca3c2) to head (85246f1).
Report is 25 commits behind head on master.

Files	Patch %	Lines
...com/networknt/schema/ValidationMessageHandler.java	76.92%	3 Missing and 3 partials ⚠️
...n/java/com/networknt/schema/MaxItemsValidator.java	50.00%	3 Missing ⚠️
.../java/com/networknt/schema/MaxLengthValidator.java	75.00%	1 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1060      +/-   ##
============================================
- Coverage     78.90%   78.88%   -0.03%     
- Complexity     1965     2018      +53     
============================================
  Files           172      189      +17     
  Lines          6352     6530     +178     
  Branches       1255     1248       -7     
============================================
+ Hits           5012     5151     +139     
- Misses          867      907      +40     
+ Partials        473      472       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.