[aggregator] efficiently GC expired aggregation versions

[rallen090]

What this PR does / why we need it:

Avoid scanning to cleanup expired agg versions.

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[bot]]

Codecov Report

Merging #3842 (f8d1b08) into master (9a2f021) will decrease coverage by 0.3%. The diff coverage is 85.2%.

@@           Coverage Diff            @@
##           master   #3842     +/-   ##
========================================
- Coverage    57.0%   56.8%   -0.3%     
========================================
  Files         552     552             
  Lines       63129   63129             
========================================
- Hits        36033   35887    -146     
- Misses      23907   24038    +131     
- Partials     3189    3204     +15     

Flag	Coverage Δ
aggregator	63.3% <90.6%> (-0.1%)	:arrow_down:
cluster	∅ <ø> (∅)
collector	58.4% <ø> (ø)
dbnode	60.4% <ø> (-0.4%)	:arrow_down:
m3em	46.4% <ø> (ø)
metrics	19.7% <0.0%> (-0.1%)	:arrow_down:
msg	74.4% <ø> (-0.1%)	:arrow_down:

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