Conditional requests

Nexus Delta supports conditional requests as defined here for the different operations: * Fetch operation for the different types of resource * Fetch original payloads for the different types of resources * Fetching the file contents

The response for those operations are augmented with a ETag response header.

The client can then use this value to set up caches and save bandwidth by using the conditional headers as Delta can immediately answer with a 304 Not Modified and not resend the full response.