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.
Instead of a linear search through pending copy-up operations, pass enough extra context to your callback so you know the oid for the copy-up completion and can thus do a direct lookup.