Ceph

make crushwrapper encode a "legacy" crushmap if (1) we're using weight sets, (2) the client is lacking features for the weight set, but (3) the weight set has a single position and no id remapping. Since these maps are only used by clients (not humans), then i don't think there's any need to preserve the original crush weights in a shadow hierarchy; we can just swpa them for the real weights and the legacy clients wll behave as expected. a helper can tell us whether this is possible so that we gate any commands that inject crush map on a compat option.