Revert "More parallellization in the batch importer"

This reverts commit fd9c7eebb9206b82172676e41bdbb9bd5be4b541.

Conflicts:
	community/kernel/src/main/java/org/neo4j/unsafe/impl/batchimport/ParallellBatchImporter.java
	community/kernel/src/test/java/org/neo4j/unsafe/impl/batchimport/ParallelBatchImporterTest.java
Merge pull request #2727 from davidegrohmann/2.1-parallel-batch-import-opt-revert

Revert "More parallellization in the batch importer"