[#285] blobonicza: Optimize upperPowerOfTwo()

The real reason is this:
```
pkg/local_object_storage/blobovnicza/sizes.go:36:69
    revive empty-block: this block is empty, you can remove it
```

Didn't want to make this function longer or to add `nolint`, thus this
change. To justify:
```
UpperBound/size=1-8          0.4924n ± 1%   0.2472n ± 2%  -49.80% (p=0.000 n=10)
UpperBound/size=1023-8       0.4936n ± 3%   0.2442n ± 1%  -50.52% (p=0.000 n=10)
UpperBound/size=66560-8      0.8201n ± 2%   0.2436n ± 1%  -70.29% (p=0.000 n=10)
UpperBound/size=41943040-8   6.6900n ± 5%   0.2432n ± 0%  -96.36% (p=0.000 n=10)
geomean                       1.075n        0.2446n       -77.24%
```

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/local_object_storage/blobovnicza/sizes.go

@@ -3,6 +3,7 @@ package blobovnicza
 import (
 	"encoding/binary"
 	"fmt"
+	"math/bits"
 	"strconv"
 )
 
@@ -31,11 +32,11 @@ func bucketForSize(sz uint64) []byte {
 	return bucketKeyFromBounds(upperPowerOfTwo(sz))
 }
 
-func upperPowerOfTwo(v uint64) (upperBound uint64) {
-	for upperBound = firstBucketBound; upperBound < v; upperBound *= 2 {
+func upperPowerOfTwo(v uint64) uint64 {
+	if v <= firstBucketBound {
+		return firstBucketBound
 	}
-
-	return
+	return 1 << bits.Len64(v-1)
 }
 
 func (b *Blobovnicza) incSize(sz uint64) {

pkg/local_object_storage/blobovnicza/sizes_test.go

@@ -1,6 +1,7 @@
 package blobovnicza
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -36,3 +37,13 @@ func TestSizes(t *testing.T) {
 		require.Equal(t, bucketKeyFromBounds(item.upperBound), bucketForSize(item.sz))
 	}
 }
+
+func BenchmarkUpperBound(b *testing.B) {
+	for _, size := range []uint64{1, 1023, 65 * 1024, 40 * 1024 * 1024} {
+		b.Run(fmt.Sprintf("size=%d", size), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				_ = upperPowerOfTwo(size)
+			}
+		})
+	}
+}