fix(upload): [OCISDEV-1477] create a new file in one write in PrepareUpload

[f-firas]

This PR cuts down the disk work the upload coordinator does when an upload finishes. Since uploads started going through the coordinator, finishing a new file took four separate writes to the node's metadata plus two saves of the upload session, where the old upload path did one of each. Every write rewrites the whole .mpk, which is why the seed phase of the load test was noticeably slower than master.

PrepareUpload already writes a batch of attributes to the node, so it now also creates the node when it doesn't exist yet, and sets the processing flag in that same batch. That makes the coordinator's separate TouchFile and MarkProcessing calls unnecessary, and the session only needs saving once, after PrepareUpload. The node is created under the id we already mint at initiate, so nothing has to be swapped in afterwards. If PrepareUpload fails, it purges the node it created itself (no Delete permission needed), and the checks that can reject an upload without touching the node, like checksums and request metadata, now run before anything is created.

[kw-security]

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