Fix flaky ParallelBatchImporterTest

- Commit "More parallelization in the batch importer" merged in again.
- Fixed randomly failing ParallelBatchImporterTest. Failures were caused by the fact that all IO threads used cyclic buffer (Ring class) as pool of ByteBuffers. Ring relied on the fact that buffers were returned in the same order that they were borrowed. This ordering was broken because multiple threads used the same Ring.