OPENDJ-1893 (CR-6600) Do not hardcode Importer threadCount to 1

Problem was due to multiple threads sharing the same Persistit exchanges. Generally speaking DBs do not like mixing transactions and threads.
By using transactions for each DB access, the problem disappears.
Are we losing any performance by using full fledged transactions? Could we somehow do these operations in a thread safe manner without the need to use transactions?

Importer.java:
Use transactions for read/write operations on DN cache.