[m3ninx] Add support for document compression

[prateek]

• Adds underpinnings to support document data compression in m3ninx.
• Support for three codecs: snappy, deflate and lz4
• Minor refactoring to abstract away docs.Reader and docs.Writer type away from the fst.Writer, into the encoding/docs package. It allows the fst.Writer to defer knowledge of the compression internals based on the CompressionOptions knob passed down.

NB: Putting this up despite not being ready to be used in production (as it is currently) to break up PRs. The feature is disabled so landing won't affect any releases. More PRs to follow.

[codecov[bot]]

Codecov Report

Merging #1646 (1ce857c) into master (79ee9f5) will decrease coverage by 0.2%. The diff coverage is 59.4%.

@@           Coverage Diff            @@
##           master   #1646     +/-   ##
========================================
- Coverage    72.3%   72.1%   -0.2%     
========================================
  Files         962     972     +10     
  Lines       80157   80578    +421     
========================================
+ Hits        57967   58169    +202     
- Misses      18398   18582    +184     
- Partials     3792    3827     +35     

Flag	Coverage Δ
aggregator	82.4% <ø> (ø)
cluster	85.7% <ø> (ø)
collector	63.9% <ø> (ø)
dbnode	80.0% <ø> (-0.1%)	:arrow_down:
m3em	73.2% <ø> (ø)
m3ninx	72.0% <59.4%> (-2.0%)	:arrow_down:
m3nsch	51.1% <ø> (ø)
metrics	17.6% <ø> (ø)
msg	74.7% <ø> (ø)
query	67.9% <ø> (ø)
x	86.4% <ø> (ø)

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[robskillington]

Nice(!)