Boxing a service normally means using `tower::util::BoxService`. That
doesn't implement `Clone` however so normally I had been combining it
with `Buffer` to get that.
But recently I discovered https://github.com/dtolnay/dyn-clone which
makes it possible to clone trait objects. So this adds a new internal
utility called `CloneBoxService` which replaces the previous
`BoxService` + `Buffer` combo in `BoxRoute`.
I'll investigate upstreaming that to tower. I think it makes sense there
since box + clone is quite a common need.
