WIP [dbnode] Fix races in retrieveRequest

[linasm]

What this PR does / why we need it: Attempts to fix two race conditions in retrieveRequest which I discovered while troubleshooting an integration test locally.

Special notes for your reviewer: Not completely sure if this is a proper fix. Struggling to understand the interleaving of req.finalized and req.resultWg.Wait() in the original code.

Does this PR introduce a user-facing and/or backwards incompatible change?: NONE

Does this PR require updating code package or user-facing documentation?: NONE

[codecov[bot]]

Codecov Report

Merging #2966 (59248bf) into master (04540bf) will decrease coverage by 0.1%. Report is 936 commits behind head on master. The diff coverage is 66.6%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2966     +/-   ##
========================================
- Coverage    72.4%   72.4%   -0.1%     
========================================
  Files        1075    1075             
  Lines       99070   99069      -1     
========================================
- Hits        71764   71763      -1     
+ Misses      22348   22346      -2     
- Partials     4958    4960      +2     

Flag	Coverage Δ
aggregator	76.3% <ø> (ø)
cluster	85.2% <ø> (ø)
collector	84.3% <ø> (ø)
dbnode	78.9% <66.6%> (+<0.1%)	:arrow_up:
m3em	74.4% <ø> (ø)
m3ninx	73.1% <ø> (-0.1%)	:arrow_down:
metrics	19.9% <ø> (ø)
msg	74.2% <ø> (-0.1%)	:arrow_down:
query	67.4% <ø> (ø)
x	80.2% <ø> (ø)

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[linasm]

@vdarulis I agree with your comments. I should have opened this as a draft as it is not ready for review yet. Changing to draft now.