Remove upload chunks only if push was successful

[varmar05]

Remove uploaded chunks not after they were used but only if whole push transaction was successful.

Fix the issue if client (v2) wants to reuse uploaded chunks after some failure. It also solves the issue of one chunk being used for more than one file (kind of edge case of the same file with different names).

[coveralls]

Pull Request Test Coverage Report for Build 16986284909

Details

• 18 of 18 (100.0%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 91.894%

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Files with Coverage Reduction	New Missed Lines	%
server/mergin/sync/models.py	1	97.02%

Totals
Change from base Build 16932482056:	0.02%
Covered Lines:	7471
Relevant Lines:	8130

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💛 - Coveralls