[Tile-serving] [openstreetmap/osm2pgsql] New "flex" backend (#1036)

[Jochen Topf]

@openstreetmap-tiles The whole point of this effort is that you can do things like proper highway shields efficiently *without* having to have extra tables in your database. So I think if this all pans out the way I envision it now, this should be able to solve your problem. Allowing tables without any geometry was a small change, so I have added this to the code.

I have added an example `highway-shields.lua` that demonstrates how to a) get refs from the route relations into the ways (in a comma-separated list) and b) put route relations into their own table and add their ids to the member ways (in an array). Here is a rendering of that data. The "shields" you are seeing are rendered from the way geometries, but have the refs from the relations. Note the `E22` and `A1` refs on the bottom. I'll leave it to you to come up with better rendering. :-)

![route-refs-as-shields](https://user-images.githubusercontent.com/113756/71271093-71fec080-2353-11ea-8e6e-88cbf831d11d.png)


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