This allows to known the full datalayer behaviour without needing
to load all the data, including the zoom from and to (new settings),
but also the color for example.
This will help also understanding datalayers usage and making
stats.
But no data migration is provided, it's retrocompatible (data
migration in OSM FR servers would be huge, so let's see if it's
really needed).
If-Match relies on ETag, which depends on the Content-Encoding,
which is more fragile given we updated the etag on save, while normal
files are served by nginx. So this may occurs false mismatch.
