Removing fsync from our local storage engine

Working with files is hard [1], and most of the complicity is from the fsync API. I am glad it can be eliminated from a kv storage engine.

[1] https://news.ycombinator.com/item?id=42805425

Author here. This is not a general argument against fsync; the design depends on SSD-only deployment, preallocated files, O_DIRECT, single-key atomicity, and device write guarantees.