[Tile-serving] [openstreetmap/osm2pgsql] Make it easier to reproduce --hstore behavior with flex backend (#1123)

[Jochen Topf]

There is one useful function `grab_tag` which pulls one tag out of the tags table which simplifies the content of your for-loop. See the `data-types.lua` example on how to use this.

Apart from that this looks pretty okay to me. I'd probably start with a list of text column names only, build the columns config for the define_table from that and then iterate over that using `grab_tag` to pull out the tags. Something like this (untested):

```
text_columns = {'foo', ...}

point_columns = {
    { column = 'geom', type = 'point' },
    { column = 'tags', type = 'hstore' }
}

for i,c in ipairs(text_columns) do
    table.insert(point_columns, { column = c, type = 'text' }
end

tables.point = osm2pgsql.define_table{
    name = 'planet_osm_point',
    ids = { type = 'node', id_column = 'osm_id' },
    columns = point_columns
}

function osm2pgsql.process_node(object)
    if next(object.tags) == nil then
        return
    end
    output_cols = { tags = object.tags }
    for c in text_columns do
        output_cols[c] = object:grab_tag(c)
    done
    tables.point:add_row(output_cols)
end
```

The `compatible.lua` was written to be compatible with the C transform, which doesn't have hstore in it if I remember correctly. But I think it would be great to have a `compatible-c.lua` and `compatible-lua.lua` example/test configuration to mimic the C transform as well as the default lua transform.

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