fix: make Event thread-safe with kevent

[hulxv]

Fix #1905

[Darksonn]

Can we also add a test that mio::event::Event implements Send/Sync somewhere? That way, whenever we add a new platform, we have to at least think about whether it should be Send/Sync if it isn't already.

[hulxv]

Can we also add a test that mio::event::Event implements Send/Sync somewhere? That way, whenever we add a new platform, we have to at least think about whether it should be Send/Sync if it isn't already.

Is that fine?

https://github.com/tokio-rs/mio/pull/1906/files#diff-40119742cba31026e7fe170d6ad1d05c82b31f302f335386f39142a1df7c023dR944-R973

[Thomasdezeeuw]

@hulxv you code didn't compile so I send some commits to fix that and apply some of the suggestions I was going to make anyway. I'll merge this and release this today.

Thanks for your contribution!