Parallel REFRESH MATERIALIZED VIEW and CTAS for AO/AOCS

[avamingli]

Make the SELECT part of REFRESH parallel for AO/AOCS storage MATERIALIZED VIEW.
Make the SELECT part of CREATE TABLE AS parallel for AO/AOCS storage table.

Parallel processes couldn't have writeable operations, assertions like below are added by PG:
'cannot update tuples during a parallel operation'.
It's not a problem for PG as workers are launched by Gather node, and the SELECT part of Refresh MV/CTAS could be parallel.
However, AO/AOCS will require batches of Row Numbers generated from gp_fastquence which will in-place update catalog. And CBDB will EnterParallelMode() anyway when ExecutePlan in QE if there is parallel across the whole plan.

Use EnterParallelMode() only for the slices who have multiple parallel workers, in theory, slices execute the SELECT part of a parallel plan.

Performance

(Only one time test on QingYun Cloud)

Plan	Refresh AO MatV	Refresh AOCO MatV	CTAS AO	CTAS AOCO
non-parallel	6.18	5.91	6.56	6.06
parallel(4 workers)	2.83	2.81	2.37	2.48

DDL:

create table t1(c1 int, c2 int) with(parallel_workers=4) distributed by(c1);
insert into t1 select i, i+1 from generate_series(1, 10000000)i;
View definition:
 SELECT sum(a.c1) AS c1,
    avg(b.c2) AS c2
   FROM t1 a
     JOIN t1 b ON a.c1 = b.c1;
Distributed by: (c1)

A Parallel Plan of CTAS for AOCS

explain(costs off) create table ctas_aoco using ao_column as select sum(a.c2) as c2, avg(b.c1) as c1 from t_p a join t_p b on a.c1 = b.c1 distributed by(c2);
                           QUERY PLAN
----------------------------------------------------------------
 Redistribute Motion 1:3  (slice1; segments: 1)
   Hash Key: (sum(a.c2))
   ->  Finalize Aggregate
         ->  Gather Motion 12:1  (slice2; segments: 12)
               ->  Partial Aggregate
                     ->  Parallel Hash Join
                           Hash Cond: (a.c1 = b.c1)
                           ->  Parallel Seq Scan on t_p a
                           ->  Parallel Hash
                                 ->  Parallel Seq Scan on t_p b
 Optimizer: Postgres query optimizer
(11 rows)

Authored-by: Zhang Mingli avamingli@gmail.com

closes: #ISSUE_Number

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[avamingli]

close #115

[avamingli]

Rebased && fix some regression cases.

[my-ship-it]

LGTM

[foreyes]

LGTM

[avamingli]

Pushed, thanks for review~