OPENDJ-2575 VLV indexed search with offset type control does not give expected results

The patch is a two man's effort, most Yannick's than mine, actually.
The counter index managed internally from VLV was not correctly populated during import/rebuild, since it is not a Tree known by the entry container.
Manage VLVIndex with a specific importer task, as other special indexes do, creating a single key after rebuild/import.