Remove re-slicing for multipart

[smallhive]

Closes #843

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 1.91388% with 615 lines in your changes are missing coverage. Please review.

Project coverage is 26.92%. Comparing base (aadef93) to head (127dd3e). Report is 1 commits behind head on master.

Files	Patch %	Lines
api/layer/multipart_upload.go	0.00%	265 Missing :warning:
api/layer/neofs_mock.go	8.54%	102 Missing and 5 partials :warning:
internal/neofs/tree.go	0.00%	86 Missing :warning:
api/layer/tree_mock.go	0.00%	55 Missing :warning:
api/layer/object.go	0.00%	49 Missing and 1 partial :warning:
internal/neofs/neofs.go	0.00%	47 Missing :warning:
api/layer/layer.go	33.33%	3 Missing and 1 partial :warning:
api/handler/acl.go	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #931 +/- ## ========================================== - Coverage 27.97% 26.92% -1.06% ========================================== Files 87 87 Lines 13590 14129 +539 ========================================== + Hits 3802 3804 +2 - Misses 9371 9903 +532 - Partials 417 422 +5 ```

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[smallhive]

Rebased

[cthulhu-rider]

main commit with reslicing removal contains only issue ref which is good for GH but not enough for git history overall. Lets write some text about motivation and used approach

[smallhive]

What happens if we're to reupload a part?

Everything will be broken. We collect full object hash moving from part to part. If you re-upload any part in the middle, we will have to re-read all parts after it, recalculate hashes, and re-upload parts

[roman-khimov]

Everything will be broken. We collect full object hash moving from part to part. If you re-upload any part in the middle, we will have to re-read all parts after it, recalculate hashes, and re-upload parts

Cool. We need to do this. And that's where per-part IDs can become handy because

Part numbers can be any number from 1 to 10,000, inclusive. A part number uniquely identifies a part and also defines its position within the object being created. If you upload a new part using the same part number that was used with a previous part, the previously uploaded part is overwritten.

And btw,

Each part must be at least 5 MB in size, except the last part.

[smallhive]

Everything will be broken. We collect full object hash moving from part to part. If you re-upload any part in the middle, we will have to re-read all parts after it, recalculate hashes, and re-upload parts

Cool. We need to do this. And that's where per-part IDs can become handy because

Part numbers can be any number from 1 to 10,000, inclusive. A part number uniquely identifies a part and also defines its position within the object being created. If you upload a new part using the same part number that was used with a previous part, the previously uploaded part is overwritten.

And btw,

Each part must be at least 5 MB in size, except the last part.

Added logic for re-upload parts