Unify OSD::send_msg_osd_{client,cluster}.

For the moment, this change just makes these routines
equivalent, removing the opportunity for incorrect reply
behavior if send_msg_osd_cluster is anywhere used in
a client reply.

By definition, the idiom to send a message to an osd by
id is safe, but it may form a rate limiter for infiniband
clients currently, since inter-cluster communications are
only over TCP.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>