[aggregator] More struct alignment fixes

[vdarulis]

What this PR does / why we need it:

Rearrange a few more common structs to reduce gc scan/memory overhead, based on fieldalignment tool

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

Merging #3908 (dc53a6b) into master (08209d4) will increase coverage by 0.0%.
The diff coverage is 83.3%.

@@          Coverage Diff           @@
##           master   #3908   +/-   ##
======================================
  Coverage    56.6%   56.6%           
======================================
  Files         553     553           
  Lines       63278   63278           
======================================
+ Hits        35836   35849   +13     
+ Misses      24241   24235    -6     
+ Partials     3201    3194    -7     

Flag	Coverage Δ
aggregator	62.3% <83.3%> (+<0.1%)	⬆️
cluster	∅ <ø> (∅)
collector	58.4% <ø> (ø)
dbnode	60.3% <ø> (+<0.1%)	⬆️
m3em	46.4% <ø> (ø)
metrics	19.7% <ø> (ø)
msg	74.4% <ø> (+<0.1%)	⬆️

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[ryanhall07]

thanks! will be interested to see how this helps with heap/gc

[vdarulis]

thanks! will be interested to see how this helps with heap/gc

It's usually minor, but since every aggregation/entry/etc. overhead is static per-metric, shaving 8-32 bytes here and there helps - last pass (on aggregator.Entry et al) did drop mem usage by ~3-4% iirc.