- Implemented OAI resumption tokens for ListRecords requests. Implements

SF feature request #620658.
- Modified DB schema for items.  Now items have a 'last_modified' property,
  which is updated whenever the item is modified (obviously.)  This date is
  used for the harvesting.
- Optimised Item.java/DCType.java:  Now, DCType.java loads in all DC types
  up front so that Item.java does not have to do more queries to get the
  elements and qualifiers.  Greatly reduces number of SQL queries per harvest.
- Added upgrade script to set last_modified date for items that need them.


git-svn-id: http://scm.dspace.org/svn/repo/trunk@644 9c30dcfa-912a-0410-8fc2-9e0234be79fd

dspace/etc/database_schema.sql

@@ -191,7 +191,7 @@ CREATE TABLE Item
   submitter_id    INTEGER REFERENCES EPerson(eperson_id),
   in_archive      BOOL,
   withdrawn       BOOL,
-  withdrawal_date VARCHAR(21)
+  last_modified   TIMESTAMP
 );
 
 -------------------------------------------------------