[#1328] pilorama: Do not skip items in SortedByFilename

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>

pkg/local_object_storage/pilorama/heap.go

@@ -2,6 +2,8 @@ package pilorama
 
 import (
 	"container/heap"
+	"slices"
+	"strings"
 )
 
 type heapInfo struct {
@@ -28,9 +30,10 @@ func (h *filenameHeap) Pop() any {
 
 // fixedHeap maintains a fixed number of smallest elements started at some point.
 type fixedHeap struct {
-	start *string
-	count int
-	h     *filenameHeap
+	start  *string
+	sorted bool
+	count  int
+	h      *filenameHeap
 }
 
 func newHeap(start *string, count int) *fixedHeap {
@@ -44,20 +47,39 @@ func newHeap(start *string, count int) *fixedHeap {
 	}
 }
 
+const amortizationMultiplier = 5
+
 func (h *fixedHeap) push(id MultiNode, filename string) bool {
 	if h.start != nil && filename <= *h.start {
 		return false
 	}
-	heap.Push(h.h, heapInfo{id: id, filename: filename})
-	if h.h.Len() > h.count {
-		heap.Remove(h.h, h.h.Len()-1)
+
+	*h.h = append(*h.h, heapInfo{id: id, filename: filename})
+	h.sorted = false
+
+	if h.h.Len() > h.count*amortizationMultiplier {
+		slices.SortFunc(*h.h, func(a, b heapInfo) int {
+			return strings.Compare(a.filename, b.filename)
+		})
+		*h.h = (*h.h)[:h.count]
 	}
 	return true
 }
 
 func (h *fixedHeap) pop() (heapInfo, bool) {
-	if h.h.Len() != 0 {
-		return heap.Pop(h.h).(heapInfo), true
+	if !h.sorted {
+		slices.SortFunc(*h.h, func(a, b heapInfo) int {
+			return strings.Compare(a.filename, b.filename)
+		})
+		if len(*h.h) > h.count {
+			*h.h = (*h.h)[:h.count]
+		}
+		h.sorted = true
+	}
+	if len(*h.h) != 0 {
+		info := (*h.h)[0]
+		*h.h = (*h.h)[1:]
+		return info, true
 	}
 	return heapInfo{}, false
 }