Add a utility to poll multiple futures simultaneously

If we have a handful of futures we want to make progress on
simultaneously we need some way to poll all of them in series,
which we add here in the form of `MultiFuturePoller`. Its probably
not as effecient as some of the options in the `futures` crate, but
it is very trivial and not that bad.

lightning/src/util/async_poll.rs

@@ -0,0 +1,34 @@
+//! Somse utilities to make working with std Futures easier
+
+use crate::prelude::*;
+use core::future::Future;
+use core::marker::Unpin;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+pub(crate) struct MultiFuturePoller<F: Future<Output = ()> + Unpin>(pub Vec<Option<F>>);
+
+impl<F: Future<Output = ()> + Unpin> Future for MultiFuturePoller<F> {
+	type Output = ();
+	fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+		let mut have_pending_futures = false;
+		for fut_option in self.get_mut().0.iter_mut() {
+			let mut fut = match fut_option.take() {
+				None => continue,
+				Some(fut) => fut,
+			};
+			match Pin::new(&mut fut).poll(cx) {
+				Poll::Ready(()) => {},
+				Poll::Pending => {
+					have_pending_futures = true;
+					*fut_option = Some(fut);
+				},
+			}
+		}
+		if have_pending_futures {
+			Poll::Pending
+		} else {
+			Poll::Ready(())
+		}
+	}
+}

lightning/src/util/mod.rs

@@ -30,6 +30,7 @@ pub mod base32;
 pub(crate) mod base32;
 
 pub(crate) mod atomic_counter;
+pub(crate) mod async_poll;
 pub(crate) mod byte_utils;
 pub(crate) mod transaction_utils;
 pub(crate) mod time;