fix(ServiceAccount): Make cache behavior more intuitive.

Now the cache is only checked for a token when the future is polled, not
at future creation time.

This also allows for reverting c2b41c3.

examples/test-svc-acct/src/main.rs

@@ -25,15 +25,12 @@ fn main() {
             println!("token is: {:?}", tok);
             Ok(())
         });
-    let mut sa2 = sa.clone();
-    let all = fut
-        .then(move |_| {
-            sa2.token(["https://www.googleapis.com/auth/pubsub"].iter())
-                .and_then(|tok| {
-                    println!("cached token is {:?} and should be identical", tok);
-                    Ok(())
-                })
-        })
-        .then(|_| Ok(()));
+    let fut2 = sa
+        .token(["https://www.googleapis.com/auth/pubsub"].iter())
+        .and_then(|tok| {
+            println!("cached token is {:?} and should be identical", tok);
+            Ok(())
+        });
+    let all = fut.join(fut2).then(|_| Ok(()));
     tokio::run(all)
 }