A GIS implementation for rocksdb.

Summary:
    This patch stores gps locations in rocksdb.

    Each object is uniquely identified by an id. Each object has
    a gps (latitude, longitude) associated with it. The geodb
    supports looking up an object either by its gps location
    or by its id. There is a method to retrieve all objects
    within a circular radius centered at a specified gps location.

Test Plan: Simple unit-test attached.

Reviewers: leveldb, haobo

Reviewed By: haobo

CC: leveldb, tecbot, haobo

Differential Revision: https://reviews.facebook.net/D15567