Remove Sync requirement from response bodies (#440)

As we learned [in Tonic] bodies don't need to be `Sync` because they can
only be polled from one thread at a time.

This changes axum's bodies to no longer require `Sync` and makes
`BoxBody` an alias for `UnsyncBoxBody<Bytes, axum::Error>`.

[in Tonic]: https://github.com/hyperium/tonic/issues/117

src/handler/mod.rs

@@ -281,8 +281,8 @@ where
     S::Future: Send,
     T: 'static,
     ReqBody: Send + 'static,
-    ResBody: http_body::Body<Data = Bytes> + Send + Sync + 'static,
-    ResBody::Error: Into<BoxError> + Send + Sync + 'static,
+    ResBody: http_body::Body<Data = Bytes> + Send + 'static,
+    ResBody::Error: Into<BoxError>,
 {
     type Sealed = sealed::Hidden;