Replace dep futures-util with helpers::FuturesResolver (#765)

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>

crates/binstalk/src/helpers.rs

@@ -1,3 +1,4 @@
+pub mod futures_resolver;
 pub mod jobserver_client;
 pub mod signal;
 pub mod tasks;