[aggregator] - fix bug where expired datapoints can be included in aggregations

[rallen090]

What this PR does / why we need it:

Datapoints outside the buffer past window are marked as expired and should not be included in aggregations. However, this was not the behavior due to a bug where the GC of expired values only clears the datapoint preceding the latest expired datapoint, and the aggregations did not account for this.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

Does this PR require updating code package or user-facing documentation?:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.8%. Comparing base (b8d9b0f) to head (6a0ca55).
⚠️ Report is 369 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #3780     +/-   ##
========================================
- Coverage    57.0%   56.8%   -0.3%     
========================================
  Files         552     552             
  Lines       63009   63013      +4     
========================================
- Hits        35969   35844    -125     
- Misses      23850   23967    +117     
- Partials     3190    3202     +12     

Flag	Coverage Δ
aggregator	63.3% <100.0%> (+<0.1%)	⬆️
cluster	∅ <ø> (∅)
collector	58.4% <ø> (ø)
dbnode	60.5% <ø> (-0.3%)	⬇️
m3em	46.4% <ø> (ø)
metrics	19.7% <ø> (ø)
msg	74.2% <ø> (-0.2%)	⬇️

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