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

[Jochen Topf]

The current version of the PR is now in pretty good shape. It doesn't do everything we might ever want (no multiple geometry columns yet, for instance), but it is getting towards something that might me mergeable. There is even some [documentation](https://github.com/joto/osm2pgsql/blob/flex-backend/flex-config/README.md) and lots of examples in the same directory.

There are still things to do and to decide though. The main points are:
* Does it work on updates in all cases? My tests look all good, but I fear there might still be corner cases.
* How to share data between first and second stage? This is currently done by serializing a magic lua variable (`osm2pgsql.userdata`) where you store everything you need in the second pass, and de-serializing it in all threads running the second pass. This can need quite a lot of memory, but it is very flexible. If we can narrow down typical use cases, we might be able to find a better solution here. (Also the current solution depends on the MessagePack Lua library which is not necessarily available everywhere.)

If you are interested in this issue, now would be a good time to test this and give me feedback.

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