Fix the bug with serialization machinery.

[danolivo]

Change the AQO plan node behaviour: now we add such node also in the case when forced stat gathering option is enabled even in disabled mode.

Also:

1. Do not add AQO info into an explain AQO sentence if AQO node isn't existed for the node.
2. Exclude node which aren't predicted by AQO from a cardinality error computation.

Introduce 'parallel_workers' regression test.