Geospatial ('2dsphere') indexes suggestions

The query evaluator is now able to detect if a query might be more efficient with a geospatial index. If so, it evaluates the existing indexes. If it decides that there is no optimal geospatial index available, it suggests one.

Currently the implementation covers only '2dsphere' geospatial indexes. Implementing the same feature for '2d' indexes is going to be my next task.