[dbnode] ReadBits to use optimized Read path

[rallen090]

What this PR does / why we need it:

The ReadBits path originally always made individual ReadByte calls for each 8 bit sequence. However, Read path is optimized to read multiple full bytes in a row if the iterator is aligned (i.e. no remaining current bits) and so we use that path for ReadBits instead now.

PRE

POST

PRE
6164           1940648 ns/op

POST
6813           1746332 ns/op

(1940648 - 1746332)/1940648 = ~10%

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?:

[arnikola]

Looks good, what kind of difference does it make?

edit: looked at the graph, seems substantial

[codecov]

Codecov Report

Merging #2205 into master will increase coverage by <.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #2205     +/-   ##
========================================
+ Coverage    72.4%   72.4%   +<.1%     
========================================
  Files        1022    1022             
  Lines       88865   88904     +39     
========================================
+ Hits        64386   64415     +29     
- Misses      20162   20167      +5     
- Partials     4317    4322      +5

Flag	Coverage Δ
#aggregator	82% <0%> (-0.1%)	⬇️
#cluster	85.3% <0%> (ø)	⬆️
#collector	82.8% <0%> (ø)	⬆️
#dbnode	79.1% <0%> (ø)	⬆️
#m3em	59.3% <0%> (-15.1%)	⬇️
#m3ninx	77.7% <0%> (+3.3%)	⬆️
#m3nsch	19.6% <0%> (-31.5%)	⬇️
#metrics	17.5% <0%> (ø)	⬆️
#msg	74.8% <0%> (-0.2%)	⬇️
#query	68.9% <0%> (ø)	⬆️
#x	49.6% <0%> (-33.6%)	⬇️

---

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[codecov]

Codecov Report

Merging #2205 into master will decrease coverage by 1.7%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master   #2205     +/-   ##
========================================
- Coverage    72.0%   70.2%   -1.8%     
========================================
  Files        1022     983     -39     
  Lines       88793   86281   -2512     
========================================
- Hits        63966   60650   -3316     
- Misses      20505   21500    +995     
+ Partials     4322    4131    -191     

Flag	Coverage Δ
#aggregator	71.1% <ø> (-11.1%)	⬇️
#cluster	85.3% <ø> (ø)
#collector	82.8% <ø> (+11.4%)	⬆️
#dbnode	74.0% <100.0%> (-4.9%)	⬇️
#m3em	57.1% <ø> (-16.3%)	⬇️
#m3ninx	68.2% <ø> (-4.2%)	⬇️
#m3nsch	51.1% <ø> (ø)
#metrics	17.6% <ø> (ø)
#msg	74.9% <ø> (+1.9%)	⬆️
#query	68.4% <ø> (ø)
#x	75.9% <ø> (-7.3%)	⬇️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 07e7df8...d2c9c98. Read the comment docs.

re-evaluating numbers, double checking this is working as intended

[robskillington]

LGTM