A default constructed one is of little use except for being a placeholder. None of the members (except for move assignment) may be called on a default

`on_close` is called before user's completion is called, `on_cleanup` is called after user's callback.

If there are less than `buffer_size` objects in buffer, writes are completed immediately. Note that this raises a caveat: we actually notify the user

Total number of bytes written.

@brief: 效验合 @param: 协议头+协议体的sendbuf, sendbuf长度 */

Sub-class can do custom pre-check if needed.

It's not specified whether pending writes are flushed or dropped after this method returns. (@sa: `WriteLast`, which guarantees a flush.) Until the `

Recursion boundary, when there's no element left.