Delete snapshot file after each successful upload

[adejanovski]

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Following up on a suggestion made in #734, we could optimize disk space usage by deleting uploaded files as we go instead of clearing the whole snapshots at the end of the backup.
If backups take a long time to get uploaded, there's a chance sstables will get compacted and the corresponding snapshot file will get persisted to disk and potentially fill the disks.

Once a file is successfully uploaded, we have no need to keep it around because even in the case the backup fails, the snapshot will get cleared and a new one will get created for any subsequent backup.

Definition of Done

• snapshot files are deleted right after successful upload, during the backup operation progress

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