radosgw-admin: bucket sync status handles missing full status

radosgw-admin: bucket sync status handles missing full status

if the full sync status object is missing, it's possible that we just
haven't started syncing it again after upgrading from just the per-shard
status objects

in this case, as long as we have a log generation 0, assume that we just
haven't initialized the full status object and try to read the gen=0
per-shard incremental status for comparison

Signed-off-by: Casey Bodley <cbodley@redhat.com>