[Experimental][DBnode Client] - Optimize fetch operations by avoiding by deduplicating replicas with identical streams

[richardartoul]

What this PR does / why we need it: This P.R has the potential to significantly decrease the CPU load of decoding compressed blocks in the M3DB client which is especially important for Protobuf workloads where decoding the stream is very expensive.

Fixes # N.A

Special notes for your reviewer: N.A

Does this PR introduce a user-facing and/or backwards incompatible change?: No this would be a transparent performance increase

Improve M3DB client decoding logic to deduplicate replicas with identical streams on a block-by-block basis, significantly reducing decoding overhead in many cases.

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

[codecov[bot]]

Codecov Report

Merging #2068 into master will increase coverage by 2.4%. The diff coverage is 54%.

@@           Coverage Diff            @@
##           master   #2068     +/-   ##
========================================
+ Coverage    62.6%     65%   +2.4%     
========================================
  Files         996    1004      +8     
  Lines       86851   86261    -590     
========================================
+ Hits        54386   56119   +1733     
+ Misses      28192   25957   -2235     
+ Partials     4273    4185     -88

Flag	Coverage Δ
#aggregator	24% <ø> (-39.3%)	:arrow_down:
#cluster	77.1% <ø> (-8.5%)	:arrow_down:
#collector	19.8% <ø> (-29%)	:arrow_down:
#dbnode	70.1% <54%> (+3.5%)	:arrow_up:
#m3em	68.4% <ø> (+12.6%)	:arrow_up:
#m3ninx	70.9% <ø> (+24.1%)	:arrow_up:
#m3nsch	51.1% <ø> (-48.9%)	:arrow_down:
#metrics	17.7% <ø> (-82.3%)	:arrow_down:
#msg	74.7% <ø> (-25.3%)	:arrow_down:
#query	61.2% <ø> (-8.4%)	:arrow_down:
#x	77.2% <ø> (+0.8%)	:arrow_up:

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[robskillington]

This is cool, let us know if there's any easy numbers to share - would be interesting.

[robskillington]

Also wonder if it's worth us sending over checksums to the client so client doesn't have to calculate checksums itself

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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