There are some buckets we access almost always, to check whether an
object is alive. In search we also iterate over lots of objects, and
`tx.Bucket()` shows itself a lot in pprof.
```
goos: linux
goarch: amd64
pkg: git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/metabase
cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
                          │      1      │                  2                   │
                          │   sec/op    │    sec/op     vs base                │
Select/string_equal-8       4.753m ± 6%   3.969m ± 14%  -16.50% (p=0.000 n=10)
Select/string_not_equal-8   4.247m ± 9%   3.486m ± 11%  -17.93% (p=0.000 n=10)
Select/common_prefix-8      4.163m ± 5%   3.323m ±  5%  -20.18% (p=0.000 n=10)
Select/unknown-8            3.557m ± 3%   3.064m ±  8%  -13.85% (p=0.001 n=10)
geomean                     4.158m        3.445m        -17.15%

                          │      1       │                  2                   │
                          │     B/op     │     B/op      vs base                │
Select/string_equal-8       2.250Mi ± 0%   1.907Mi ± 0%  -15.24% (p=0.000 n=10)
Select/string_not_equal-8   2.250Mi ± 0%   1.907Mi ± 0%  -15.24% (p=0.000 n=10)
Select/common_prefix-8      2.250Mi ± 0%   1.907Mi ± 0%  -15.24% (p=0.000 n=10)
Select/unknown-8            2.243Mi ± 0%   1.900Mi ± 0%  -15.29% (p=0.000 n=10)
geomean                     2.248Mi        1.905Mi       -15.26%

                          │      1      │                  2                  │
                          │  allocs/op  │  allocs/op   vs base                │
Select/string_equal-8       56.02k ± 0%   47.03k ± 0%  -16.05% (p=0.000 n=10)
Select/string_not_equal-8   56.02k ± 0%   47.03k ± 0%  -16.05% (p=0.000 n=10)
Select/common_prefix-8      56.02k ± 0%   47.03k ± 0%  -16.05% (p=0.000 n=10)
Select/unknown-8            55.03k ± 0%   46.04k ± 0%  -16.34% (p=0.000 n=10)
geomean                     55.78k        46.78k       -16.12%
```

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

@@ -0,0 +14,4 @@
+	return &bucketCache{}
+}
+
+func lockedBucket(bc *bucketCache, tx *bbolt.Tx) *bbolt.Bucket {