Expose drafter (endpoint) version as a HTTP response header on drafter queries

[RickMoynihan]

As part of the cache race work PR #522 we added a new drafter:version triple to endpoints and draftsets. This is currently exposed on the swagger rest API.

It would be useful to also surface this as extra metadata in the HTTP response header on SPARQL queries. My initial thoughts are that this should probably be an ETag.

I think for consistency API routes may also want to copy the version into an ETag, to more generically at the protocol layer support proxies etc.

The reasoning for this is that currently you'd need to make a request to the API for the version, then query it, by which point the version may have changed.

Use cases are:

1. Reliably knowing the endpoint version that a SPARQL query result was generated from for caching / invalidation etc.
2. Being able to cache API requests, e.g. data dumps where we return a whole endpoints contents as RDF.
3. Consistency - Muttnik and many applications need to make multiple queries. In many scenarios it is useful to know nothing in the endpoint has changed between these queries; i.e. all queries were made against the same database value and the generated result is consistent.

Additionally:

• SPARQL update requests to drafter should return the new drafter:version as an ETag after they have run
• Drafter client should provide a means to obtain the version of any SPARQL result / request (we should break this out into a new issue).