Remove buffer from BoxRoute (#270)

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.

src/lib.rs

@@ -1004,7 +1004,6 @@
 #[macro_use]
 pub(crate) mod macros;
 
-mod buffer;
 mod error;
 mod json;
 mod util;