Benchmark results:
```
goos: linux
goarch: amd64
pkg: git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/pilorama
cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
│ old │ new │
│ sec/op │ sec/op vs base │
ForestSortedIteration/bbolt,root-8 207.2µ ± 6% 173.6µ ± 6% -16.23% (p=0.000 n=10)
ForestSortedIteration/bbolt,leaf-8 3.910µ ± 5% 3.928µ ± 7% ~ (p=0.529 n=10)
geomean 28.46µ 26.11µ -8.27%
```
They are not representative, as the worst case is when we have multiple
items of different lengths. However, `FileName` is usually less than 100
in practice, so the asymptotics is the same.
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
