Use brace_style for async blocks

[Iron-E]

I again apologize for how many issues I have opened at once. I just started using rustfmt on my project and have noticed a lot of things about brace_style, as an avid "AlwaysNextLine"-er.

async blocks don't adhere to brace_style. For example, the code snippet below is shown when running rustfmt:

async {
  // stuff
}

It should look like this:

async
{
  // stuff
}

[calebcartwright]

Closing as per #4938 (comment)