Replace remaining direct uses of futures with futures_util

Slight downscope which helps combat the antipattern which is the futures glob
crate. While futures_util is still a large crate, it has better defaults and
is smaller by virtue of not pulling the executor.

substrate/node/Cargo.toml

@@ -22,7 +22,7 @@ name = "serai-node"
 [dependencies]
 clap = { version = "4", features = ["derive"] }
 
-futures = "0.3"
+futures-util = "0.3"
 jsonrpsee = { version = "0.16", features = ["server"] }
 
 sp-core = { git = "https://github.com/serai-dex/substrate" }

substrate/node/src/service.rs

@@ -1,6 +1,6 @@
 use std::{boxed::Box, sync::Arc};
 
-use futures::stream::StreamExt;
+use futures_util::stream::StreamExt;
 
 use sp_timestamp::InherentDataProvider as TimestampInherent;
 use sp_consensus_babe::{SlotDuration, inherents::InherentDataProvider as BabeInherent};