mon: set forwarded message recv stamp

mon: set forwarded message recv stamp

Set it to the stamp of the MForward that carried us.  One could argue
we really want the original receive stamp on the origin, but that is
not available to us, and this is better than nothing.

In particular, this gives 'ceph log ...' commands a timestamp when they
are forwarded via a peon.  The stamp is still between when the request
is sent and when it is committed/acked, so all is well from the
client's perspective.

Signed-off-by: Sage Weil <sage@inktank.com>