parallel distinct union and aggregate support patch

Hi hackers,

I write a path for soupport parallel distinct, union and aggregate using batch sort.

steps:

 1. generate hash value for group clauses values, and using mod hash value save to batch

 2. end of outer plan, wait all other workers finish write to batch

 3. echo worker get a unique batch number, call tuplesort_performsort() function finish this batch sort

 4. return row for this batch

 5. if not end of all batchs, got step 3

BatchSort paln make sure same tuple(group clause) return in same range, so Unique(or GroupAggregate) plan can work.

path 2 for parallel aggregate, this is a simple use

but regress failed for partitionwise aggregation difference plan

from GatherMerge->Sort->Append->...

to  Sort->Gahter->Append->...

I have no idea how to modify it.

Same idea I writed a batch shared tuple store for HashAgg in our PG version, I will send patch for PG14 when I finish it.