Improve ChunkAppend parallel worker assignment

[svenklemm]

The initial iteration of assigning multiple workers per subplan
would always assign a fixed amount of workers per child. This
patch changes the logic to assign each child one worker and then
moving to the next subplan, iterating until all subplans are finished.
This will lead to a much better distribution of workers especially
when the children are not well balanced.

[cevian]

Could we just point out any remaining differences in the commit msg with core PG logic, or if there arent any point that out too.

[codecov]

Codecov Report

Merging #1465 into master will decrease coverage by 8.82%.
The diff coverage is 91.17%.

@@            Coverage Diff             @@
##           master    #1465      +/-   ##
==========================================
- Coverage   90.22%   81.39%   -8.83%     
==========================================
  Files         117      117              
  Lines       17610    16896     -714     
==========================================
- Hits        15888    13753    -2135     
- Misses       1722     3143    +1421

Flag	Coverage Δ
#cron	81.39% <91.17%> (-8.83%)	⬇️
#pr	81.39% <91.17%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
src/planner.c	86.11% <100%> (-5.52%)	⬇️
src/chunk_append/exec.c	95.84% <90.62%> (+0.05%)	⬆️
tsl/src/nodes/gapfill/gapfill.c	45.45% <0%> (-54.55%)	⬇️
src/chunk_insert_state.c	31.62% <0%> (-54.42%)	⬇️
src/chunk.h	0% <0%> (-50%)	⬇️
src/utils.h	0% <0%> (-50%)	⬇️
src/plan_add_hashagg.c	46.87% <0%> (-40.63%)	⬇️
src/plan_expand_hypertable.c	43.28% <0%> (-40.22%)	⬇️
src/copy.c	44.67% <0%> (-37.66%)	⬇️
src/scan_iterator.h	0% <0%> (-33.34%)	⬇️
... and 85 more

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