shifting does not happen correctly on failure scenarios

[calvinlfer]

Hello there,

I noticed that a couple of methods on ZkClient do this. For example:

def setDataOf[F[_]: Async: ContextShift](zk: ZooKeeper,node: ZkNode, data: Option[Chunk[Byte]], version: Option[Int]): F[Option[ZkStat]] =
      Async[F].async[Option[ZkStat]] { cb => zk.setData(node.path, data.map(_.toArray).orNull,version.getOrElse(-1), mkStatCallBack(cb), null) } <* implicitly[ContextShift[F]].shift

def existsNow[F[_]: Async: ContextShift](zk: ZooKeeper, node: ZkNode): F[Option[ZkStat]] =
      Async[F].async[Option[ZkStat]] { cb => zk.exists(node.path, false, mkStatCallBack(cb), null) } <* implicitly[ContextShift[F]].shift

It would be much better to use guarantee so even in case of a failure scenario, shifting will happen no matter what.