Fix/GC big objects parts

[carpawell]

Closes #2858.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 38.33333% with 37 lines in your changes missing coverage. Please review.

Project coverage is 23.69%. Comparing base (e5da072) to head (31b9074). Report is 4 commits behind head on master.

:exclamation: Current head 31b9074 differs from pull request most recent head 658cbde

Please upload reports for the commit 658cbde to get more accurate results.

Files	Patch %	Lines
pkg/services/replicator/process.go	0.00%	15 Missing :warning:
pkg/local_object_storage/shard/exists.go	0.00%	3 Missing and 1 partial :warning:
pkg/local_object_storage/shard/gc.go	42.85%	4 Missing :warning:
pkg/local_object_storage/shard/shard.go	0.00%	3 Missing :warning:
pkg/services/replicator/replicator.go	0.00%	3 Missing :warning:
pkg/core/object/object.go	66.66%	1 Missing and 1 partial :warning:
pkg/local_object_storage/engine/inhume.go	71.42%	1 Missing and 1 partial :warning:
pkg/local_object_storage/metabase/exists.go	60.00%	2 Missing :warning:
cmd/neofs-node/object.go	0.00%	1 Missing :warning:
pkg/core/object/fmt.go	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2863 +/- ## ========================================== + Coverage 23.56% 23.69% +0.12% ========================================== Files 773 770 -3 Lines 44672 44540 -132 ========================================== + Hits 10529 10555 +26 + Misses 33290 33131 -159 - Partials 853 854 +1 ```

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[carpawell]

Found corner case in shard distribution, will add tests and fix.

[carpawell]

OK, a fresh view and an extended test showed that it did not work. Let it be simple and error-proof.