Add support for Write Ahead Log

[nakabonne]

Fixes #5

This PR adds support for WAL only for insertOperation. The format is:

	   +--------+---------------------+--------+--------------------+----------------+
	   | op(1b) | len metric(varints) | metric | timestamp(varints) | value(varints) |
	   +--------+---------------------+--------+--------------------+----------------+

[codecov-commenter]

Codecov Report

Merging #28 (f3bcf01) into main (2ad4f71) will decrease coverage by 3.34%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
- Coverage   66.08%   62.74%   -3.33%     
==========================================
  Files          19       19              
  Lines         955     1111     +156     
==========================================
+ Hits          631      697      +66     
- Misses        229      288      +59     
- Partials       95      126      +31     

Impacted Files	Coverage Δ
memory_partition.go	81.68% <33.34%> (-1.65%)	⬇️
storage.go	52.23% <46.52%> (-3.86%)	⬇️
disk_wal.go	51.05% <51.05%> (ø)
wal.go	50.00% <60.00%> (-50.00%)	⬇️

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