Adds support for geo-bounds filtering in geogrid aggregations

[talevy]

It is fairly common to filter the geo point candidates in
geohash_grid and geotile_grid aggregations according to some
viewable bounding box. This change introduces the option of
specifying this filter directly in the tiling aggregation.

This is even more relevant to geo_shape where the bounds will restrict
the shape to be within the bounds

this optional bounds parameter is parsed in an equivalent fashion to
the bounds specified in the geo_bounding_box query.

example:

POST /museums/_search?size=0
{
    "aggregations" : {
        "tiles-in-bounds" : {
            "geohash_grid" : {
                "field" : "location",
                "precision" : 8,
                "bounds": {
                  "top_left" : "53.4375, 4.21875",
                  "bottom_right" : "52.03125, 5.625"
                }
            }
        }
    }
}

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Geo)

[nknize]

Overall it looks good. Left a few minor thoughts and suggestions. At minimum I think we should use the Lucene test utilities where we can. Separate from that I think we can introduce a little more randomization to the tests.

[iverase]

I think this is almost there, just want to think if we should use different implementations for the bounded and unbounded case when iterating doc values.

[iverase]

lgtm

[talevy]

@elasticmachine update branch

thanks Ignacio!

[talevy]

@elasticmachine update branch