Wip copy on read

An improved copy-on-read implementation based on the pull request (https://github.com/ceph/ceph/pull/2158) by Min Chen <minchen@ubuntukylin.com>, Li Wang <liwang@ubuntukylin.com> and Yunchuan Wen <yunchuanwen@ubuntukylin.com>. 

The bufferlist holding the entire object from parent is allocated separately.

Also provided a unit test for verifying the implementation.
Why the "1" for the result code?  Are you just attempting to re-run the state machine to send the (copy-up +) write op?