chore(client): re-enable client's custom executor config

src/client/conn.rs

@@ -453,16 +453,15 @@ impl Builder {
         }
     }
 
-    /*
     /// Provide an executor to execute background HTTP2 tasks.
     pub fn executor<E>(&mut self, exec: E) -> &mut Builder
     where
-        E: Executor<Box<dyn Future<Item=(), Error=()> + Send>> + Send + Sync + 'static,
+        for<'a> &'a E: tokio_executor::Executor,
+        E: Send + Sync + 'static,
     {
         self.exec = Exec::Executor(Arc::new(exec));
         self
     }
-    */
 
     pub(super) fn h1_writev(&mut self, enabled: bool) -> &mut Builder {
         self.h1_writev = enabled;