[Tile-serving] [openstreetmap/mod_tile] Expire using metatiles zoom levels? (Issue #447)

[Hummeltech]

It seems like that is basically what is already being done, more or less, here's an example command:
```shell
printf "16/56715/4908\n18/226860/19632\n19/453726/39265\n" | \
  render_expired --min-zoom 3 --max-zoom 19 --verbose --touch-from 0
```
1. The first line is read (`16/56715/4908`) and a loop is started on it for zoom levels 3 to 19 (`min-zoom` to `max-zoom`):
    ```YAML
    ** Message: 17:48:11.546: Read valid line: 16/56715/4908
    ** Message: 17:48:11.546: Starting loop on 19/453720/39264 for zoom levels 3 to 19
    ```
    1. Zoom levels 19-6 are `requested`, and the metatiles do not exist on disk, so they are `skipped`
        ```YAML 
        ** Message: 17:48:11.546: Processing: 19/453720/39264
        ** Message: 17:48:11.546: Skipping 'file:///var/cache/renderd/tiles/default/19/96/233/201/86/128.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 18/226860/19632
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/18/48/116/108/43/128.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 17/113430/9816
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/17/16/178/182/21/8.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 16/56715/4908
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/16/0/209/211/130/136.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 15/28357/2454
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/15/0/96/233/201/0.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 14/14178/1227
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/14/0/48/116/108/8.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 13/7089/613
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/13/0/16/178/182/0.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 12/3544/306
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/12/0/0/209/211/128.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 11/1772/153
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/11/0/0/96/233/136.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 10/886/76
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/10/0/0/48/116/8.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 9/443/38
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/9/0/0/16/178/128.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 8/221/19
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/8/0/0/0/209/128.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 7/110/9
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/7/0/0/0/96/136.meta' (metatile does not exist)
        ** Message: 17:48:11.547: Processing: 6/55/4
        ** Message: 17:48:11.547: Skipping 'file:///var/cache/renderd/tiles/default/6/0/0/0/48/0.meta' (metatile does not exist)
        ```
    1. Zoom levels 5-3 are `requested`, and the metatiles do exist, so they are `touched`
        ```YAML 
        ** Message: 17:48:11.547: Processing: 5/27/2
        ** Message: 17:48:11.547: Touching 'file:///var/cache/renderd/tiles/default/5/0/0/0/16/128.meta'
        ** Message: 17:48:11.547: Processing: 4/13/1
        ** Message: 17:48:11.547: Touching 'file:///var/cache/renderd/tiles/default/4/0/0/0/0/128.meta'
        ** Message: 17:48:11.547: Processing: 3/6/0
        ** Message: 17:48:11.547: Touching 'file:///var/cache/renderd/tiles/default/3/0/0/0/0/0.meta'
        ```
1. The second line is read (`18/226860/19632`) and nothing needs to be done as this metatile and all below it have already been `requested`:
    ```YAML
    ** Message: 17:48:11.547: Read valid line: 18/226860/19632
    ** Message: 17:48:11.547: Starting loop on 19/453720/39264 for zoom levels 3 to 19
    ** Message: 17:48:11.547: Already requested metatile containing '19/453720/39264', moving on to next input line
    ```
1. Then the third line is read (`19/453726/39265`) and again nothing needs to be done as this metatile and all below it have already been `requested`:
    ```YAML
    ** Message: 17:48:11.547: Read valid line: 19/453726/39265
    ** Message: 17:48:11.547: Starting loop on 19/453726/39265 for zoom levels 3 to 19
    ** Message: 17:48:11.547: Already requested metatile containing '19/453726/39265', moving on to next input line
    ```

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