support for selecting attributes in query

- add cursor/feature/queryplan support
- implement reduced features in FeatureHandler JSON response
- implement reduced features in BaseRenderer as computed by query
  and styles
- implement natively in geopkg

@jdeolive, this ended up being a bit nastier/messier than anticipated. It unfortunately shows only a minor performance improvement. My guess is the approach taken in the SelectFieldsCursor (copying fields) is part of the problem. I'll play with the latter and see what happens but any feedback appreciated.