Drop the old tarsum computation for Docker <0.10

This landed in 82021b4 (Merging bypass-tarsum, 2014-04-07, #300), but
those Dockers are now ancient history.

I'm against this sort of user-agent-dependent behavior in general.  If
we want to version the API, it makes sense to have version checks in
one place (e.g. in the URL, or in an explicit Docker-Version header,
or in the PUT payload, ...). I'm happy with either the URL approach
(always explicit) or the Docker-Version approach (defaulting to the
current version). If we make some small change to the API (such as not
storing layer tarsums), and want to add per-feature headers or
registry-side options to enable/disable this behaviour, that's
fine. If we can auto-detect new features (e.g. if
Docker-Checksum-Payload exists, it must be using the new SHA-256
semantics), that's fine too. But regardless of how we version the API,
conflating the API version with the user-agent version just seems
confusing.

Olivier wants us to warn older clients instead of just failing when
their tarsum doesn't match our checksum [1], so we still need to use
their user-agent string to decide which clients get warned.  I've
softened the old DockerVersion into a more relaxed
docker_client_version, which no longer rejects non-Docker clients [2].
If we can't version the API, we can at least use a blacklist to deny
old implementations instead of a whitelist that requires other clients
to fake the Docker-daemon User-Agent string.

For the 'PUT /v1/images/<image_id>/checksum' endpoint, we don't need
to rely on the user-agent, because the old X-Docker-Checksum was
replaced by the new X-Docker-Checksum-Payload.  If the client isn't
setting X-Docker-Checksum-Payload, just say that in the error message
and hint that it might be due to an outdated client.

I'd like to remove all the user-agent-dependent behavior, but we're
currently also using it to setup some tag metadata in
docker_registry.tags.put_tag.  I don't touch that here, since I
haven't tracked down the reasoning behind that behavior (although I
suspect it's also a bad idea ;).

I also haven't touched the user-agent bits under tests/, since I
haven't checked to see if they're designed to test this check, or to
test the put_tags check, or just to bypass either of those checks.

[1]: https://github.com/docker/docker-registry/pull/570#issuecomment-62963997
[2]: https://github.com/docker/docker-registry/issues/375