Replicas sharing a database need one place to agree on the Last-Modified
of rewritten metadata, which process memory cannot give them.
- Add metadata_dates, keyed by ecosystem, proxy URL and package, holding the
ETag being dated and its date in Unix seconds, so no zone is involved
- Insert a package's first date only if no replica recorded one first
- Swap a date only if it is still the one read, so of two replicas
switching a package at once only one succeeds
- Create it in both schemas and in migration 011
