Add Send + Sync check to all services (#277)

Should prevent us accidentally introducing breaking changes.

Fixes https://github.com/tokio-rs/axum/issues/275

src/tests/mod.rs

@@ -721,3 +721,5 @@ where
 pub(crate) fn assert_send<T: Send>() {}
 pub(crate) fn assert_sync<T: Sync>() {}
 pub(crate) fn assert_unpin<T: Unpin>() {}
+
+pub(crate) struct NotSendSync(*const ());