OPENDJ-2016 Implement new on disk merge import strategy based on storage engine


OnDiskMergeStorageImporter.java:
Removed DNCacheImpl + implemented Dn2IdDnCache which requires an EntryID in the insert() method.
Used Importer.
Some changes consequence of adding entryID to OnDiskMergeBufferImporter.DNCache.insert().
Removed more unused code.

DN2ID.java:
Added insert(WriteableTransaction, DN, EntryID), required by the new OnDiskMergeStorageImporter.Dn2IdDnCache.insert().

OnDiskMergeBufferImporter.java:
In DNCache.insert(), now accept a entryID parameter, required by the new OnDiskMergeStorageImporter.Dn2IdDnCache.insert().
Added Dn2IdDnCache, alternative implementation of DNCache.
Moved ImportTask.insert() to Dn2IdDnCache.insert().

Suffix.java:
In isParentProcessed(), moved the check on dn2id to the new OnDiskMergeBufferImporter.Dn2IdDnCache.contains() (avoids coupling with WriteableTransaction) + removed several unused parameters