DS-1711 DS-1712 DS-1466 Implement consistent definition of Private items

I have opened a feature branch on DSpace to allow easy review and improvement of the proposed patch to fix our lacking definition of private item.
https://wiki.duraspace.org/display/~bollini/DSpace+Item+state+definitions

The initial PR has been tested using both SOLR Browse DAO and PostgreSQL DAO (the Oracle porting has been done and it was very trivial so probably no need to test it...).
The following tests have been performed:
- item submission as a generic submitter without workflow enabled using both XMLUI and JSPUI marking the item as private
- make an existent item as private
- make public an existent private item
- widthdrawn and reinstantiate an item

What is missing?
We need to agree about the upgrade path for users that could have private items in their database. My first idea is to wirte a simple sql update statement to set withdrawn=false, in_archive= true for all the discoverable=false item that are not in workflow/workspace.
With this change the item will be unsearcheable and unaccessible to all, as in the previous version make an item private also withdrawn the item so no resoucepolicy are present for such items