- Extend the querytype methods offered by KeywordIndex (add Gt, Lt, etc).

- Add data structures to return docids_count(), indexed_count() and
  not_indexed_count() more efficiently if these methods get used frequently.

- Ensure we can do resultset.sort(i1).sort(i2).sort(i3) in order to sort by i1
  first, then i2, then i3.  Requires sort stability guarantees.

- Do less (ideally no) work during a reindex in the text index when nothing has
  changed.

