[dbnode] Decode avoid checked bytes in segment reader

[rallen090]

What this PR does / why we need it:

Targeting the unchecked byte reads

Looks like a ~7% improvement in the small-byte-count case (e.g. benchmark on 10 instead of 2000).

PRE
BenchmarkNextIteration/series_10-12                 2047            534476 ns/op
BenchmarkNextIteration/series_2000-12                193           9585290 ns/op
POST
BenchmarkNextIteration/series_10-12                 2197            557157 ns/op
BenchmarkNextIteration/series_2000-12                237           4950101 ns/op

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 #2190 into master will decrease coverage by 0.9%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #2190    +/-   ##
=======================================
- Coverage    72.2%   71.3%    -1%     
=======================================
  Files        1019    1019            
  Lines       88430   88433     +3     
=======================================
- Hits        63916   63116   -800     
- Misses      20234   20982   +748     
- Partials     4280    4335    +55

Flag	Coverage Δ
#aggregator	82% <ø> (-0.1%)	⬇️
#cluster	85.2% <ø> (-0.2%)	⬇️
#collector	82.8% <ø> (ø)	⬆️
#dbnode	76.8% <100%> (-2.3%)	⬇️
#m3em	74.4% <ø> (ø)	⬆️
#m3ninx	74.6% <ø> (-0.2%)	⬇️
#m3nsch	51.1% <ø> (ø)	⬆️
#metrics	17.6% <ø> (ø)	⬆️
#msg	74.9% <ø> (ø)	⬆️
#query	68.1% <ø> (ø)	⬆️
#x	83.4% <ø> (+0.2%)	⬆️

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[arnikola]

LGTM

[robskillington]

LGTM