Expire using metatiles zoom levels?

[joto]

When a tile is expired on, say, zoom level 19, it is marked in the expire list in that zoom level 19. But really we are using metatiles, so always the whole metatile is getting expired. If there are a lot of tiles in the same metatile, they are all in the expire list. In the worst case there are 8x8=64 times the number of entries in the expire list than we would really need. We could just write out expire list on zoom level 16 instead.

So I propose some kind of option to render_expires (and maybe render_list) that says: interpret zoom levels as meta tile zoom levels, not tile zoom levels.

See also: https://github.com/osm2pgsql-dev/osm2pgsql/issues/2190