ForestDB: Reimplement geo-queries

[snej]

To restore geo-query functionality on iOS, and support adding it on other platforms, we'll need to implement it on top of ForestDB.

A full implementation would require r-trees, but Volker says we can get pretty far by using a regular index whose keys are (x,y) pairs encoded in a format like Geohash that collates lexicographically.

[snej]

I did some work on this a month or so ago, enough to get basic point-in-rect queries working, but not rect-intersection. Currently on hold.