Add concurrent tests using join and join_all

These tests are essentially copies of the `concurrent` test,
specifically using `join` and `join_all`.  Note that for the `join_all`
example to work, all of the `Client` clones must be created before *any*
requests are added, otherwise there will be a lifetime problem with the
second request, saying that second client, `c2`, is still borrowed when
`req1` is dropped.  It would require a larger redesign to fix this
issue.

tarpc/tests/service_functional.rs

@@ -1,6 +1,6 @@
 use assert_matches::assert_matches;
 use futures::{
-    future::{ready, Ready},
+    future::{join_all, ready, Ready},
     prelude::*,
 };
 use std::io;
@@ -10,6 +10,7 @@ use tarpc::{
     server::{self, BaseChannel, Channel, Handler},
     transport::channel,
 };
+use tokio::join;
 use tokio_serde::formats::Json;
 
 #[tarpc_plugins::service]
@@ -110,3 +111,59 @@ async fn concurrent() -> io::Result<()> {
 
     Ok(())
 }
+
+#[tokio::test(threaded_scheduler)]
+async fn concurrent_join() -> io::Result<()> {
+    let _ = env_logger::try_init();
+
+    let (tx, rx) = channel::unbounded();
+    tokio::spawn(
+        tarpc::Server::default()
+            .incoming(stream::once(ready(rx)))
+            .respond_with(Server.serve()),
+    );
+
+    let client = ServiceClient::new(client::Config::default(), tx).spawn()?;
+
+    let mut c = client.clone();
+    let req1 = c.add(context::current(), 1, 2);
+
+    let mut c = client.clone();
+    let req2 = c.add(context::current(), 3, 4);
+
+    let mut c = client.clone();
+    let req3 = c.hey(context::current(), "Tim".to_string());
+
+    let (resp1, resp2, resp3) = join!(req1, req2, req3);
+    assert_matches!(resp1, Ok(3));
+    assert_matches!(resp2, Ok(7));
+    assert_matches!(resp3, Ok(ref s) if s == "Hey, Tim.");
+
+    Ok(())
+}
+
+#[tokio::test(threaded_scheduler)]
+async fn concurrent_join_all() -> io::Result<()> {
+    let _ = env_logger::try_init();
+
+    let (tx, rx) = channel::unbounded();
+    tokio::spawn(
+        tarpc::Server::default()
+            .incoming(stream::once(ready(rx)))
+            .respond_with(Server.serve()),
+    );
+
+    let client = ServiceClient::new(client::Config::default(), tx).spawn()?;
+
+    let mut c1 = client.clone();
+    let mut c2 = client.clone();
+
+    let req1 = c1.add(context::current(), 1, 2);
+    let req2 = c2.add(context::current(), 3, 4);
+
+    let responses = join_all(vec![req1, req2]).await;
+    assert_matches!(responses[0], Ok(3));
+    assert_matches!(responses[1], Ok(7));
+
+    Ok(())
+}