`X-Datalayer-Version` and `X-Datalayer-Reference` are now used instead
of the `Last-Modified` and `If-Unmodified-Since` headers.
`Last-Modified` is granular to the second, which led to problems with
the versionning. The new system uses timestamps instead.
This commit also changes the way versions were created. Previously,
the associated version was coming from two different places:
the last modified time from the filesystem and a `time.time()` call
done when saving the model, which could result in the two getting
out of sync.
This can be helpful in situations where the name of the index is not known, as it can be with pre 1.0 deployed instance.
This commit also generates the UUIDs directly using an SQL statement.
The server tries to merge conflicting saves of
the same layer.
What it does:
- use the `If-Unmodified-Since` header to check
if changes happened to the stored data ;
- Compare the incoming version with its reference version
to get a diff.
- Reapply the diff on top of the latest version.
- If the merge is not possible, return a
"422 Conflict" HTTP response.
- If the merge worked, return the merged document,
to be updated by the client.
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.
