Allow tagging bits for outer join marking in the hash table. Prepare an
iterator that can later be used by the HashTableSource.

This commit wraps up outer join support and support for TPC-H Q13.
We allow inducing a continuation pipeline after the outer join.

That continuation pipeline starts with a HashTableSource for the outer
join, and then feeds into the remaining suboperators of the pipeline.

Only let one thread produce the non-matched tuples.