Etag support on storage entities

[PhilipSkinner]

Ensure that etags are supported on storage entities, supporting both the response of the version with the etag header and the use of if-match headers on:

• GET
• POST
• PUT
• PATCH
• DELETE

On modification operations this is to deal with modification conflicts and should return a 409 Conflict status code.

On fetching a single resource, the if-match header can be used to determine if the resource has changed - if it has not then a 304 Not Modified is to be returned.

The use of * as the if-match header value means either "ignore the version identifier" or "fetch the latest" - depending upon the incoming HTTP request method.

If the if-match header is omitted then it is to be as though the client sent the value * within it.