* Adapt `retrieve_image` to also return a timestamp based on the
CDN's last modified time; adapt other methods accordingly
* For the maps module, use `chrono::Utc` instead of
`tokio::time::Instant` and use `chrono::Duration` instead of
`tokio::time::Duration`
* Pass the maps timestamp to the `sample` function so it can use
that timestamp as base
* Use `Position` everywhere instead of latitude/longitude float values
* Implement `Partial`, `Eq` and `Hash` for `Position` so it can
part of a cache key
* Drop the `cache_key` helper function
* Rename the `address_position` function to `resolve_address`
* Add methods on `Position` for formatting latitude/longitude with
a given precision (used for URL parameters in providers)
Paul van Tilburg