Use Zipfile comment for tags

[zerebubuth]

We support adding key-value tags to stored S3 objects, however this can be unexpectedly costly and we've turned it off by default. The ability to trace the run which generated a particular metatile is still useful, so we would like an alternative way to store tags.

One option is zipfile comments, which allows a string of up to 64kB to be stored - plenty for storing the run_id. An additional benefit is that, because this is stored in the zipfile and not by the storage system, it's portable across the filesystem and S3 implementations.