fix StreamUtil to execute the map function for each batch in the thread pool

[xiaoyu-yang-gh]

methods like filter and map on stream are lazy. They just create a new stream. Traversal of the pipeline source does not begin until the terminal operation of the pipeline is executed.

The StreamUtil wants the map method executed for each batch in the thread pool Asynchronously, so we should make the stream pipeline executed in the thread-pool for the reason mentioned above.

Here the collect is the terminal operation that triggers the stream execution.

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (5249a34) 46.19% compared to head (1debdf1) 46.18%.

Files	Patch %	Lines
...om/linkedin/avroutil1/builder/util/StreamUtil.java	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #548      +/-   ##
============================================
- Coverage     46.19%   46.18%   -0.01%     
+ Complexity     4546     4544       -2     
============================================
  Files           407      407              
  Lines         28442    28443       +1     
  Branches       4642     4642              
============================================
- Hits          13138    13137       -1     
- Misses        13730    13731       +1     
- Partials       1574     1575       +1     

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[karthikrg]

LGTM. This is a subtle behavior with flatMap that changed in Java 10.
https://blog.jooq.org/are-java-8-streams-truly-lazy-not-completely/