wip copy on read

This is the copy-on-read feature based on both cmgitdream's (https://github.com/ceph/ceph/pull/2158) and cchengleo's (https://github.com/ceph/ceph/pull/2587) improvement. Please close https://github.com/ceph/ceph/pull/3030 and https://github.com/ceph/ceph/pull/2852 as it is easier to merge into this.
It might be nice to move all exec copyup logic into the new CopyupRequest class.  In the case of a write request, once the copyup has completed, it will callback into the queued AioWrite request so that it can write its data.