Turn world state save into a zero-copy operation

Merged Adam Reichold requested to merge zero-copy-save into zero-copy-restore

As a follow-up to !110 (merged), this uses vectored I/O to turn saving the world state into zero-copy operation as well.

In a short benchmark, this reduced the time spent writing 1,536*8=12,288 snapshots totalling 19 GB from 35s to 20s.

Edited by Adam Reichold

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