This was a regression which has been fixed upstream.

This reverts commit 67e7677e.

We're no longer allowed to reserve formerly used namespaces.

- Drop the custom service file, use the packaged one
- Merge in the comments from the packaged config file

Other caches around this code use 500000 entries, so just copy that.

This cache was seeing constant evictions at its former size (70k).

With a size of 200k it now stabilizes at ~122k entries with a
significant drop in server load.

archlinux/infrastructure!473