Arvados

Part of the Keep storage classes story.

Accept X-Keep-Desired-Classes header in write requests.

• If X-Keep-Desired-Classes header is missing, use class "default".
• If X-Keep-Desired-Classes header is invalid (example?), return a 422 error.
• When writing data, only write to a volume that satisfies all of the desired classes.
• When checking whether data already exists, only check volumes that satisfy all of the desired classes.