Don't bake in Send + 'static.

Send + 'static was baked in to make it possible to spawn futures onto
the default executor. We can accomplish the same thing by offering
helper fns that do the spawning while not requiring it for the rest of
the functionality.

Fixes https://github.com/google/tarpc/issues/212

example-service/src/server.rs

@@ -75,11 +75,11 @@ async fn main() -> io::Result<()> {
         // the generated World trait.
         .map(|channel| {
             let server = HelloServer(channel.as_ref().as_ref().peer_addr().unwrap());
-            channel.respond_with(server.serve())
+            channel.respond_with(server.serve()).execute()
         })
         // Max 10 channels.
         .buffer_unordered(10)
-        .for_each(|_| futures::future::ready(()))
+        .for_each(|_| async {})
         .await;
 
     Ok(())