[multistage] improve dispatch exception handling

[walterddr]

• dispatch/server pair is already tested in the QueryServer test
• utilize the submission service just like the one used in QueryServer/QueryDispatcher which is the center of the dispatch/server pair

[Jackie-Jiang]

This is no longer a service. IMO it might be more readable if we directly implement this logic in QueryServer.submit(). I don't really see the value of this wrapper class

[walterddr]

good point. will convert it into a util --> its only job is to maintain the list of futures for completion (wrapper class will make it easy to retest the logic IMO)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.03%. Comparing base (e6655dd) to head (47e5f49).
⚠️ Report is 3179 commits behind head on master.

Files with missing lines	Patch %	Lines
...apache/pinot/query/service/server/QueryServer.java	84.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #11688      +/-   ##
============================================
- Coverage     63.10%   63.03%   -0.07%     
+ Complexity     1121     1118       -3     
============================================
  Files          2343     2342       -1     
  Lines        125693   125663      -30     
  Branches      19310    19318       +8     
============================================
- Hits          79315    79217      -98     
- Misses        40728    40796      +68     
  Partials       5650     5650              

Flag	Coverage Δ
integration	<0.01% <0.00%> (ø)
integration1	<0.01% <0.00%> (ø)
integration2	0.00% <0.00%> (ø)
java-11	63.01% <84.61%> (-0.04%)	⬇️
java-17	62.89% <84.61%> (-0.02%)	⬇️
java-20	49.94% <84.61%> (-13.00%)	⬇️
temurin	63.03% <84.61%> (-0.07%)	⬇️
unittests	63.03% <84.61%> (-0.07%)	⬇️
unittests1	67.19% <84.61%> (-0.05%)	⬇️
unittests2	14.44% <0.00%> (-0.04%)	⬇️

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[Jackie-Jiang]

Can this ever throw Throwable? Or is it always wrapped in ExecutionException?

[walterddr]

it shouldn't be but it would be hell of a pain to debug if it did. so just to keep it safe. we should at least have a branch to handle it