Revert "Wait: wait without callback, otherwise there will be use-after-dtor."

This reverts commit 7b16ce6770.

That patch is invalid.

src/Watcher.cpp

@@ -107,7 +107,7 @@ void etcd::Watcher::Wait(std::function<void(bool)> callback)
   currentTask.then([this, callback](pplx::task<void> const & resp_task) {
     resp_task.wait();
     callback(this->stubs->call->Cancelled());
-  }).wait();
+  });
 }
 
 void etcd::Watcher::Cancel()