GeoHash Filter
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Previous versions of the GeoPointFieldMapper just stored the actual geohash
of a point. This commit changes the behavior of storing geohashes by storing
the geohash and all its prefixes in decreasing order in the same field. To
enable this functionality the option geohash_prefix must be set in the mapping.

This behavior allows to filter GeoPoints by their geohashes. Basically a
geohash prefix is defined by the filter and all geohashes that match this
prefix will be returned. The neighbors flag allows to filter geohashes
that surround the given geohash cell. In general the neighborhood of a
geohash is defined by its eight adjacent cells.

To enable this, the type of filtered fields must be geo_point with geohashes
and geohash_prefix enabled.

For example:
    curl -XPUT 'http://127.0.0.1:9200/locations/?pretty=true' -d '{
        "mappings" : {
            "location": {
                "properties": {
                    "pin": {
                        "type": "geo_point",
                        "geohash": true,
                        "geohash_prefix": true
                    }
                }
            }
        }
    }'

This example defines a mapping for a type location in an index locations
with a field pin. The option geohash arranges storing the geohash of
the pin field.

To filter the results by the geohash a geohash_cell needs to be defined.
For example
    curl -XGET 'http://127.0.0.1:9200/locations/_search?pretty=true' -d '{
        "query": {
            "match_all":{}
        },
        "filter": {
            "geohash_cell": {
                "field": "pin",
                "geohash": "u30",
                "neighbors": true
            }
        }
    }'

This filter will match all geohashes that start with one of the following
prefixes: u30, u1r, u32, u33, u1p, u31, u0z, u2b and u2c.

Internally the GeoHashFilter is either a simple TermFilter, in case no
neighbors should be filtered or a BooleanFilter combining the TermFilters
of the geohash and all its neighbors.

Closes #2778