Add MOSDRepOp and MOSDRepOpReply

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>