Make the coverage save function async

[chkeita]

          It might be worthwhile (as a forever backlog feature) to add `AsyncWrite` support for `write_xml`, that way we can use tokio's `BufWriter`. I'm not sure how much time we spend blocking the thread to write these coverage files but I guess if they get big enough there's some value there. Especially since we write the file every time we merge the coverage which I think is on every input processed?

Originally posted by @tevoinea in #3410 (comment)

AB#164551