Loosen the restriction outer path has Motion of parallel plan

[avamingli]

We drop all outer paths who have Motion of parallel plan to avoid deadlock when mix parallel-aware hashjoin with parallel-oblivious paths.

And we fix it to enable_parallel, which is stricter than needed.
It's possible to keep such a path when enable_parallel is on and enable_parallel_hash is off.
Because we can make sure that there is no parallel-aware hashjoin, and of course, no deadlock issue like above.

By loosening the restriction to enable_parallel_hash, such parallel-oblivious plan would be possible.

explain(costs off) select * from t1 right join t2 on t1.b = t2.a;
                            QUERY PLAN
------------------------------------------------------------------
 Gather Motion 6:1  (slice1; segments: 6)
   ->  Hash Left Join
         Hash Cond: (t2.a = t1.b)
         ->  Redistribute Motion 6:6  (slice2; segments: 6)
               Hash Key: t2.a
               Hash Module: 3
               ->  Parallel Seq Scan on t2
         ->  Hash
               ->  Redistribute Motion 3:6  (slice3; segments: 3)
                     Hash Key: t1.b
                     Hash Module: 3
                     ->  Seq Scan on t1
 Optimizer: Postgres query optimizer
(13 rows)

Before this change, we could only get a non-parallel plan when enable_parallel=on and enable_parallel_hash=off

explain(costs off) select * from t1 right join t2 on t1.b = t2.a;
                            QUERY PLAN                            
------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)
   ->  Hash Right Join
         Hash Cond: (t1.b = t2.a)
         ->  Redistribute Motion 3:3  (slice2; segments: 3)
               Hash Key: t1.b
               ->  Seq Scan on t1
         ->  Hash
               ->  Redistribute Motion 3:3  (slice3; segments: 3)
                     Hash Key: t2.a
                     ->  Seq Scan on t2
 Optimizer: Postgres query optimizer
(11 rows)

Authored-by: Zhang Mingli avamingli@gmail.com

closes: #ISSUE_Number

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[avamingli]

This causes SemiJoin plan diffs, need to dig.

[avamingli]

This causes SemiJoin plan diffs, need to dig.

Fixed.