SQL: Added index on tags_vn.date
SQL: Added index on tags_vn.date

This index speeds up most of the reports generated using /g/links. I've
also tested other index possibilities, but they weren't all that
satisfactionary:
- An index on tags_vn.uid does speed up several queries, but since
  these queries may not have to be very common, I'd like to have some
  more real-world testing before adding the index.
- Sorting on VN title is slow, regardless of any indexes on vn_rev.title
  or tags_vn.vid (not tested one on vn.latest, which could possibly
  help...). I have seen a few queries that were faster with an index on
  tags_vn.vid, but not many and not significantly.
- An index on vn_rev.tag isn't necessary, the index for the primary key
  on (tag,vid,uid) already takes care of that. PostgreSQL is clever
  enough to be able to use that multi-column index for the tag column.

christian-roggia/vndb,morkt/vndb,morkt/vndb,erengy/vndb,christian-roggia/vndb,christian-roggia/vndb,erengy/vndb,morkt/vndb,morkt/vndb,morkt/vndb,erengy/vndb