load-only-keys version of get_range_slices

Given Keyspace ks, and an optional startOnKey string, the following snippet returns a Set of 100 keys without de-serializing the underlying columns. The tradeoff is that you have no local check on tombstones. Pass the last key retrieved as startOnKey to page.
                
ColumnParent clp = new ColumnParent(columnFamilyName);                
SlicePredicate sp = new SlicePredicate();
sp.setColumn_names(new ArrayList<byte[]>());
KeyRange keyRange = new KeyRange();
keyRange.setCount(100);

if ( StringUtils.isBlank(startOnKey)) {
    keyRange.setStart_key("");
} else {
    keyRange.setStart_key(startOnKey);
}
keyRange.setEnd_key("");
return ks.getRangeSlices(clp, sp, keyRange).keySet();