set create file option to false on opening a segment file just created

[ylgrgyq]

Hi there,

I think maybe we can set the create option in std::fs::OpenOptions to false to open a Segment file just created

[timvisee]

Thank you for diving into this.

Looks correct to me, though I'll ask two others to review this as well.

[agourlay]

There is a Clippy lint to fix on master to be able to move forward here.

warning: usage of an `Arc` that is not `Send` or `Sync`
   --> src/mmap_view_sync.rs:150:20
    |
150 |             inner: Arc::new(UnsafeCell::new(mmap)),
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: the trait `Sync` is not implemented for `UnsafeCell<MmapMut>`
    = note: required for `Arc<UnsafeCell<MmapMut>>` to implement `Send` and `Sync`
    = help: consider using an `Rc` instead or wrapping the inner type with a `Mutex`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#arc_with_non_send_sync
    = note: `#[warn(clippy::arc_with_non_send_sync)]` on by default

[timvisee]

Thanks for this change @ylgrgyq!