ParallelBatchImporterTest should have a test for the writer factory enabled by default

Details:
 - make sure we test the default configuration in ParallelBatchImporterTest
 - refactor to allow to test all the possible configuration
 - disable the test with IoQueue due to a known issue in parallelizing I/O writes
 - make package protected the constructor used only for testing
Improved comments in doImport
Make Funnel independent from IoQueue
Improved test
Remove unused File from the WriteFactory.create

- cleanup IoQueueTest
Merge pull request #3096 from davidegrohmann/2.1-fix-pbi-tests

[2.1] ParallelBatchImporterTest should have a test for the writer factory enabled by default