Sort parts by number and server creation time

[smallhive]

Closes #901.

During my first review, I didn't make two fields sort, only one - this was an error point.

Runed

S3TEST_CONF=s3tests.conf tox -- s3tests_boto3/functional/test_s3.py::test_multipart_resend --disable-warnings

Before changes

80 failed, 70 passed in 408.67s (0:06:48)

After changes

150 passed in 402.04s (0:06:42)

[codecov[bot]]

Codecov Report

Attention: 32 lines in your changes are missing coverage. Please review.

Comparison is base (205f942) 33.68% compared to head (72d5830) 33.60%. Report is 5 commits behind head on master.

Files	Patch %	Lines
internal/neofs/tree.go	0.00%	12 Missing :warning:
api/layer/multipart_upload.go	0.00%	11 Missing :warning:
api/layer/tagging.go	0.00%	7 Missing :warning:
api/handler/unimplemented.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #929 +/- ## ========================================== - Coverage 33.68% 33.60% -0.08% ========================================== Files 64 64 Lines 10834 10859 +25 ========================================== Hits 3649 3649 - Misses 6777 6802 +25 Partials 408 408 ```

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