src/main/java/com/googlecode/objectify/impl/QueryImpl.java
QueryResults<Key<T>> keysIterator() {
		final QueryEngine queryEngine = loader.createQueryEngine();
		final KeyQuery query = this.actual.newKeyQuery();
		return queryEngine.queryKeysOnly(query);
	}
QueryResults<Key<T>> keysIterator() {
		final QueryEngine queryEngine = loader.createQueryEngine();
		final KeyQuery query = this.actual.newKeyQuery();
		return queryEngine.queryKeysOnly(query);
	}
Get an iterator over the keys.  Not part of the public api, but used by QueryKeysImpl.  Assumes
that setKeysOnly() has already been set.