[query] Use optional ptr ints for per query limits to allow for specifying zero to disable limits

[robskillington]

What this PR does / why we need it:

Previously setting zero would default to the 100k default max series limit which conflicts with how the header works (setting zero disables limit). This makes both behave consistently (config + headers now act the same, zero disables, omitting uses default and setting explicit number enables at that number).

Special notes for your reviewer:

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

NONE

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

NONE

[codecov]

Codecov Report

Merging #3376 (8034ce7) into master (8034ce7) will not change coverage.
The diff coverage is n/a.

❗ Current head 8034ce7 differs from pull request most recent head 872006e. Consider uploading reports for the commit 872006e to get more accurate results

@@           Coverage Diff           @@
##           master    #3376   +/-   ##
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  Coverage    72.4%    72.4%           
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  Files        1100     1100           
  Lines      102475   102475           
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  Hits        74225    74225           
  Misses      23146    23146           
  Partials     5104     5104           

Flag	Coverage Δ
aggregator	76.9% <0.0%> (ø)
cluster	84.9% <0.0%> (ø)
collector	84.3% <0.0%> (ø)
dbnode	79.0% <0.0%> (ø)
m3em	74.4% <0.0%> (ø)
m3ninx	73.5% <0.0%> (ø)
metrics	19.8% <0.0%> (ø)
msg	74.5% <0.0%> (ø)
query	67.0% <0.0%> (ø)
x	80.3% <0.0%> (ø)

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[mway]

I'd really like to stop introducing pointer values in config, especially for scalar types. Maybe we could start using a consistent convention of "0 for disable, -1 for default", or a special magic value, or something like that? Open to ideas.