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.
Minor: you only need to copy the object up to the parent overlap (not necessarily the whole object).