futures-util has too many dependencies and it contains a lot of code of
which we only use `futures_util::stream::{FuturesUnordered, StreamExt}`.
We don't even need most of the functionalities in `FuturesUnordered` as
we just need the output of first future that either returns `Err(_)` or
`Ok(Some(_))`.
So we replace it with ou own homebrew solution (~80 loc) and it's easier
to use.
Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>
