Rewrite more traits with futures using async_trait

[matko]

the LayerStore trait and its implementation have been fully rewritten with async_trait.

Contributes to #56.

[codecov]

Codecov Report

Merging #89 (30a0147) into main (41a3bd4) will decrease coverage by 0.89%.
The diff coverage is 26.63%.

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   70.23%   69.33%   -0.90%     
==========================================
  Files          32       32              
  Lines       12219    12302      +83     
  Branches     2359     2431      +72     
==========================================
- Hits         8582     8530      -52     
- Misses       1092     1161      +69     
- Partials     2545     2611      +66     

Impacted Files	Coverage Δ
src/storage/layer.rs	65.24% <ø> (-1.50%)	⬇️
src/store/mod.rs	47.83% <17.14%> (-3.17%)	⬇️
src/storage/directory.rs	58.06% <19.23%> (-0.76%)	⬇️
src/storage/cache.rs	48.45% <27.41%> (-7.73%)	⬇️
src/storage/memory.rs	76.51% <41.37%> (-1.11%)	⬇️
src/storage/delta.rs	61.84% <0.00%> (+0.52%)	⬆️
... and 1 more

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[spl]

Related to #56?

[spl]

I skimmed through everything. It looks beautiful! ❤️