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/util/clone_box_service.rs

@@ -0,0 +1,75 @@
+use futures_util::future::BoxFuture;
+use std::future::Future;
+use std::task::{Context, Poll};
+use tower::ServiceExt;
+use tower_service::Service;
+
+/// A boxed Service that implements Clone
+///
+/// Could probably upstream this to tower
+pub(crate) struct CloneBoxService<T, U, E> {
+    inner: Box<
+        dyn CloneService<T, Response = U, Error = E, Future = BoxFuture<'static, Result<U, E>>>
+            + Send,
+    >,
+}
+
+impl<T, U, E> CloneBoxService<T, U, E> {
+    pub(crate) fn new<S>(inner: S) -> Self
+    where
+        S: Service<T, Response = U, Error = E> + Clone + Send + 'static,
+        S::Future: Send + 'static,
+    {
+        let inner = Box::new(inner.map_future(|f| Box::pin(f) as _));
+        Self { inner }
+    }
+}
+
+impl<T, U, E> Clone for CloneBoxService<T, U, E> {
+    fn clone(&self) -> Self {
+        Self {
+            inner: dyn_clone::clone_box(&*self.inner),
+        }
+    }
+}
+
+impl<T, U, E> Service<T> for CloneBoxService<T, U, E> {
+    type Response = U;
+    type Error = E;
+    type Future = BoxFuture<'static, Result<U, E>>;
+
+    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        CloneService::poll_ready(&mut *self.inner, cx)
+    }
+
+    fn call(&mut self, req: T) -> Self::Future {
+        CloneService::call(&mut *self.inner, req)
+    }
+}
+
+trait CloneService<R>: dyn_clone::DynClone {
+    type Response;
+    type Error;
+    type Future: Future<Output = Result<Self::Response, Self::Error>>;
+
+    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;
+
+    fn call(&mut self, req: R) -> Self::Future;
+}
+
+impl<R, T> CloneService<R> for T
+where
+    T: Service<R> + Clone,
+{
+    type Response = T::Response;
+    type Error = T::Error;
+    type Future = T::Future;
+
+    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        Service::poll_ready(self, cx)
+    }
+
+    fn call(&mut self, req: R) -> Self::Future {
+        Service::call(self, req)
+    }
+}