With a large number of objects in a flat tree, sorting the nodes
dominates the latency of the operation. This scales nonlinearly
as O(n log n). Pairing heap has O(1) insert, and O(log n) extractMin,
which allows us to have O(n + log n) = O(n) latency on the first
operation, albeit with a slight increase in total running time.
On a real cluster with 2m objects, the latency decreased from 25s to
15s.
```
goos: linux
goarch: amd64
pkg: git.frostfs.info/TrueCloudLab/frostfs-node/pkg/services/tree
cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
│ cache │ noparent │
│ sec/op │ sec/op vs base │
GetSubTree/latency-8 2349.9µ ± 19% 572.1µ ± 17% -75.65% (p=0.000 n=10)
GetSubTree/total_time-8 70.62m ± 8% 86.02m ± 3% +21.81% (p=0.000 n=10)
geomean 12.88m 7.015m -45.54%
│ cache │ noparent │
│ B/op │ B/op vs base │
GetSubTree/latency-8 43.87Mi ± 0% 32.81Mi ± 0% -25.22% (p=0.000 n=10)
GetSubTree/total_time-8 43.87Mi ± 0% 32.81Mi ± 0% -25.22% (p=0.000 n=10)
geomean 43.87Mi 32.81Mi -25.22%
│ cache │ noparent │
│ allocs/op │ allocs/op vs base │
GetSubTree/latency-8 400.0k ± 0% 500.0k ± 0% +24.99% (p=0.000 n=10)
GetSubTree/total_time-8 400.0k ± 0% 500.0k ± 0% +24.99% (p=0.000 n=10)
geomean 400.0k 500.0k +24.99%
```
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
