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

Implemented the memory mapped files buffer for import phase 1.
Remain to copy/stream data from phase 1 buffers to trees in backend's storage.
... minus bugs of course.


OnDiskMergeStorageImporter.java:
Implemented MemoryMappedBufferImporter for importPhaseOne().
Added Id2EntryPutTask and Id2EntryData inner classes for putting in id2entry.
Renamed MigrateExistingTask to MigrateExistingEntriesTask.
Removed more unused code.