lib/cache: switch .cachelock to fcntl()
This gives us correctness, especially on "staleness" detection. For simplicity we now don't remove "stale" .cachelock on opening cache, but it doesn't obstruct us in any way (and overflow will remove it).
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This gives us correctness, especially on "staleness" detection. For simplicity we now don't remove "stale" .cachelock on opening cache, but it doesn't obstruct us in any way (and overflow will remove it).