DNM: add osdmap checksums

hrm... i'm not sure it will matter.  even if we can't encode, we can still decode.  either it is a harmless difference (but one we don't want to propagate) or we are already screwed and letting the objecter see it won't be any different.  in particular, even if we get the pristine full map from the mon, objecter won't behave any differently. 

u