Completes the IdMapper interface change

by letting InputRelationship deal with Object ids for start/end nodes.
Now it became apparent that stages 1 and 2 cannot be executed in parallel
if the IdMapper needs preparation. Put that check in and added comments
about it.
Fixes a license header
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