DNM: add osdmap checksums

I see. I wonder if this solution might be more pain than it's worth when we do get around to having extra (but not-behavior-changing) info in the maps.
The obvious alternative is to just have OSDs know that their maps don't match, and refuse to propagate them. That way the load on the monitors is simply a full map for every newly-created-or-down-for-too-long OSD, rather than every new map for every OSD with old code.