wip copy on read

This is the copy-on-read feature based on both cmgitdream's (https://github.com/ceph/ceph/pull/2158) and cchengleo's (https://github.com/ceph/ceph/pull/2587) improvement. Please close https://github.com/ceph/ceph/pull/3030 and https://github.com/ceph/ceph/pull/2852 as it is easier to merge into this.
What happens if you don't have CoR enabled and the clone object doesn't exist? You need to read from the parent to attempt to satisfy the request.