crimson/mon: keep a copy of sent MMonCommand messages

crimson/mon: keep a copy of sent MMonCommand messages

as per Yingxin Cheng,

> The send process can be asynchronous (there is a conn.out_q, or if the
> underlying socket lives in a different core in the m:n model to be
> ordered there). If user really wants to reuse a message, they must be
> careful not to modify it because it may result in modifing the pending
> messages.
>
> I think the best way is to copy the message if user want to resend it,
> and keep the ceph_assert(!msg->get_seq()). It may looks good to reuse a
> message under lossy policy, but the correctness is now up to user not to
> modify it inplace.

see also https://github.com/ceph/ceph/pull/39786

Signed-off-by: Kefu Chai <kchai@redhat.com>