Change pooling options for index array pool to prevent (almost) unbounded growth over time

[richardartoul]

[codecov]

Codecov Report

Merging #1254 into master will increase coverage by <.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1254     +/-   ##
========================================
+ Coverage    70.6%   70.7%   +<.1%     
========================================
  Files         759     759             
  Lines       63828   63832      +4     
========================================
+ Hits        45121   45153     +32     
+ Misses      15781   15757     -24     
+ Partials     2926    2922      -4

Flag	Coverage Δ
#aggregator	81.6% <ø> (ø)	⬆️
#cluster	85.6% <ø> (ø)	⬆️
#collector	78.4% <ø> (ø)	⬆️
#dbnode	80.8% <ø> (ø)	⬆️
#m3em	73.2% <ø> (ø)	⬆️
#m3ninx	75.4% <100%> (ø)	⬆️
#m3nsch	51.1% <ø> (ø)	⬆️
#metrics	18.2% <ø> (ø)	⬆️
#msg	74.9% <ø> (ø)	⬆️
#query	60.5% <ø> (ø)	⬆️
#x	76% <ø> (ø)	⬆️

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[robskillington]

LGTM