Arbiter constructor for glommio CPU pinning

[thedodd]

When the glommio feature is being used, it would be awesome to be able to construct an Arbiter and also pass through the config needed to pin the arbiter's thread to some target CPU, which is a big part of Glommio's value add. The method could take a Placement param, and a callback. The callback will be passed the LocalExecutorBuilder allowing for further customization. Matches the builder's pattern in the glommio docs.

Arbiter::new_glommio(placement, move |exc| {
  exc.name("my_arb").ring_depth(some_depth) // etc.
});

[fafhrd91]

i dont think anybody can provide change, PR is welcomed.