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
