LocalFileSystem: use read_at instead of seek + read #622

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Currently, the LocalFileSystem uses seek followed by a read, which saves a syscall per request.

Describe the solution you'd like

We should be able to use read_at instead (https://doc.rust-lang.org/std/os/unix/fs/trait.FileExt.html
It is available for unix (linux/osx / windows):

// Platform specific traits needed for read_at
#[cfg(unix)]
use std::os::unix::fs::FileExt; 
#[cfg(windows)]
use std::os::windows::fs::FileExt;

let file = Arc::new(File::open(path)?);
file.read_at(&mut buf, 500).unwrap();
...

Describe alternatives you've considered

Additional context