[aggregator] Use WaitForInterrupt to handle interrupts

[nbroyles]

What this PR does / why we need it:

WaitForInterrupt is the more idiomatic way for us to
handle interrupts and also more gracefully handles
a nil interruptCh.

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 #3676 (85c2dd6) into master (93eb67b) will decrease coverage by 0.0%.
The diff coverage is 0.0%.

@@           Coverage Diff            @@
##           master   #3676     +/-   ##
========================================
- Coverage    56.7%   56.6%   -0.1%     
========================================
  Files         551     551             
  Lines       61894   61896      +2     
========================================
- Hits        35112   35094     -18     
- Misses      23633   23647     +14     
- Partials     3149    3155      +6     

Flag	Coverage Δ
aggregator	61.4% <0.0%> (-0.1%)	⬇️
cluster	∅ <ø> (∅)
collector	58.4% <ø> (ø)
dbnode	60.6% <ø> (-0.1%)	⬇️
m3em	46.4% <ø> (ø)
metrics	19.7% <ø> (ø)
msg	74.2% <ø> (-0.1%)	⬇️

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