feat(client): Make client an optional feature

cc #2223

BREAKING CHANGE: The HTTP client of hyper is now an optional feature. To
  enable the client, add `features = ["client"]` to the dependency in
  your `Cargo.toml`.

src/client/conn.rs

@@ -25,7 +25,7 @@ use tower_service::Service;
 
 use super::dispatch;
 use crate::body::HttpBody;
-use crate::common::{task, BoxSendFuture, Exec, Future, Pin, Poll};
+use crate::common::{task, exec::{BoxSendFuture, Exec}, Future, Pin, Poll};
 use crate::proto;
 use crate::rt::Executor;
 #[cfg(feature = "http1")]