Properly support concurrent file writes

[madadam]

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Concurrent writes were disabled for simplicity - the previous implementation was buggy and non-trivial to fix. Write concurrency is currently low-priority feature (it's not needed for the mobile apps) so disabling it seems like the easiest way forward for now.

Eventually we should bring write concurrency back, but implemented it properly. Ideally we should mimic the behaviour of established, POSIX-compliant filesystems.

[IvanaBlzvc]

https://redmine.equalit.ie/issues/31630