[DOP-23969] Fix Greenplum parallel connections warning for master=local[N]

[dolfinus]

Change Summary

Java's Runtime.availableProcessors() return number of available processors in the system, but starting Spark session with master=local[N] applies the hard limit for the number of executor threads. So even if system has a lot of processor cores available, only some of them will be used by Spark.
This lead to showing a warning that the number of Greenplum connections is high even if Spark session was started with master=local[1] or master=local[10] (max 10 executors, so max 10 connections to GP, warning threshold is 30).

Fixed, added a bunch of unit tests, using mocks to cover different cases (master=local, master=yarn with static allocation, master=yarn with dynamic allocation).

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.33%. Comparing base (60aea9e) to head (079388d).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #342      +/-   ##
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+ Coverage    92.02%   92.33%   +0.31%     
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  Files          228      228              
  Lines         9867     9875       +8     
  Branches      1013     1013              
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+ Hits          9080     9118      +38     
+ Misses         599      566      -33     
- Partials       188      191       +3     

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