[Tile-serving] [openstreetmap/osm2pgsql] Rewrite tile expiry (management of expired tiles) (#709)

[Matt Amos]

> It seems that the current implementation of a tree drops some tiles. I did not investigate further because it is hidden in the old, ugly implementation. Red tiles were written to the output file by both implementations. Blue tiles were only written by my implementation.

I notice that the blue tiles appear to be twice as large as the red tiles, suggesting that they're at one zoom level lower. Is that the case? Has the output changed if only a single zoom level is requested (e.g: `--expire-tiles 16-16`)?

What confused me is this exchange:

> > > @pnorman wrote:
> > > if I'm reading your description right, the two methods are the same for outputting just a single zoom, right?
> >
> > Yes, they are not almost the same.
>
> That looks like a difference in the selection of tiles (out of scope), not subtiles.

If the two methods are not (almost) the same for outputting just a single zoom, then I think I'd want to understand the difference in the selection of tiles (more tiles, fewer tiles, by what proportion?) and the reason why before I updated to the new expiry algorithm.

Not that a change is necessarily bad, and I don't think it should hold up this issue being resolved. I think it would be really useful to have a thorough description of what's changed and what differences users can expect in the changelog / release notes.

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