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.
No matter the object is been fetched by READ or WRITE, we have to wait this object to be written on to the disk. Thus this "wait_for_complete" should not be harmful.