Improve send performance

[numinnex]

Since monoio doesn't provide async version of metadata api for File, we use spawn_blocking to perform that call (inspired by tokio). Without providing and threadpool monoio will execute those calls locally, it's better idea to cache those informations and update them atomically. It's mostly information about file size, we can easily track it.
Currently the performance of our send benchmark is on pair with the tokio version, but I think we can do better, explore options how to make send more performant.