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.
This changes error model to actually allow errors. I think if we're going to use this for things like tonic's route we need a more flexible error handling model. The same `handle_error` adaptors are still there but services aren't required to have `Infallible` as their error type. The error type is simply propagated all the way through.
David Pedersen