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.
But the difference is between asynchronously waiting (for a callback) and synchronously waiting by blocking the librados thread.  If you block the librados thread here, it won't be able to perform other work.