Fix the way to take offset so that it can decode no matter the number of metrics is

[nakabonne]

This PR does:

• write actually encoded data points for each metric
• reset encoder params used for computation (like deltas) whenever flushing
• also stop using gzip for now.

[codecov-commenter]

Codecov Report

Merging #12 (c55ac69) into main (167b85e) will increase coverage by 7.11%.
The diff coverage is 57.90%.

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   54.24%   61.35%   +7.11%     
==========================================
  Files          18       18              
  Lines         861      877      +16     
==========================================
+ Hits          467      538      +71     
+ Misses        330      259      -71     
- Partials       64       80      +16     

Impacted Files	Coverage Δ
storage.go	52.34% <27.28%> (+3.95%)	⬆️
disk_partition.go	57.41% <57.15%> (+22.12%)	⬆️
encoding.go	44.87% <72.23%> (+22.77%)	⬆️
bstream.go	84.42% <100.00%> (+3.09%)	⬆️

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