improve gracefully shutdown

[Fullstop000]

• improve gracefully shutdown
• add simple tests for recovery

Signed-off-by: Fullstop000 [email protected]

[codecov]

Codecov Report

Merging #60 into master will increase coverage by 0.55%.
The diff coverage is 89.23%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   87.80%   88.36%   +0.55%     
==========================================
  Files          34       34              
  Lines        7216     7425     +209     
==========================================
+ Hits         6336     6561     +225     
+ Misses        880      864      -16     

Impacted Files	Coverage Δ
src/record/reader.rs	95.51% <ø> (+2.09%)	⬆️
src/logger.rs	46.00% <28.57%> (-5.07%)	⬇️
src/version/version_set.rs	74.05% <78.94%> (-0.51%)	⬇️
src/db/mod.rs	85.12% <93.67%> (+4.77%)	⬆️
src/options.rs	91.11% <100.00%> (+3.23%)	⬆️
src/storage/mem.rs	92.05% <100.00%> (+0.35%)	⬆️
src/version/version_edit.rs	78.59% <100.00%> (-7.56%)	⬇️
src/record/writer.rs	87.14% <0.00%> (-9.69%)	⬇️
src/filter/bloom.rs	95.07% <0.00%> (-1.36%)	⬇️
... and 16 more

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[Fullstop000]

Relate to #3