Ensure that the iterator returned by `read_zones_from_file` is Send + Sync

[amatissart]

This will be useful for the ongoing integration of ES7 in "mimisrbrunn", that attempts to build a thread-safe futures::stream::Stream.

[remi-dupre]

For the funny story, I've just spent a few minutes to understand that I also needed this exact thing for what I am currently doing.