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.
this test case should enable the copy-on-read option similar to https://github.com/ceph/ceph/pull/2820/files#diff-72e3f904206c591a1086f3530472c6d9R2047