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]

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 ⚠️
api/layer/multipart_upload.go	0.00%	11 Missing ⚠️
api/layer/tagging.go	0.00%	7 Missing ⚠️
api/handler/unimplemented.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            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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