Handling of Dependent Objects

[lonvia]

( copied over from #1663)

We should think about ways of doing attribute-based "expire" and re-processing. One option would be to add some kind of hash to each database entry. That hash must be calculated in the Lua config based on the tags used in an object. And osm2pgsql would keep track of those hashes and make sure objects with the same hash are re-processed whenever an object with that hash changes.