Decouple schema parsing from code generation

[karthikrg]

Decouple schema parsing (fast) from code generation (slow) to be able to run avro codegen and plugins in parallel.
Remove dead code in SpecificRecordGenerator by following IDE warnings.

[codecov-commenter]

Codecov Report

Attention: 32 lines in your changes are missing coverage. Please review.

Comparison is base (39d884c) 45.99% compared to head (7e0de3a) 46.00%.

Files	Patch %	Lines
.../operations/codegen/own/AvroUtilCodeGenPlugin.java	69.23%	14 Missing and 10 partials ⚠️
...vroutil1/codegen/SpecificRecordClassGenerator.java	84.00%	1 Missing and 3 partials ⚠️
...vroutil1/builder/plugins/BuilderPluginContext.java	80.00%	2 Missing ⚠️
...s/codegen/own/AvroUtilOperationContextBuilder.java	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #532   +/-   ##
=========================================
  Coverage     45.99%   46.00%           
+ Complexity     4487     4486    -1     
=========================================
  Files           405      406    +1     
  Lines         28140    28129   -11     
  Branches       4635     4631    -4     
=========================================
- Hits          12943    12940    -3     
+ Misses        13641    13633    -8     
  Partials       1556     1556           

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