Add MOSDRepOp and MOSDRepOpReply

Add the two new message type and change the corresponding code flow as well.

Basically the idea to have MOSDRepOp is to seperate subop(read/write)
out of other subop(pull/push,etc), so that we can cleanup some unused fields in
the message type, then save some encoding/decoding overhead.

The backward compatibility is also remian, if talking with old version OSD who
doesn't support osd_client_subop/subopreply, will fall back to osd_subop/subopreply.

Sage: rename MOSDClientSubOp -> MOSDRepOp

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>
Signed-off-by: Sage Weil <sage@redhat.com>