client/finality-grandpa: Make round_communication use bounded channel (#4691)

* clinet/finality-grandpa: Make round_communication use bounded channel

`round_communication` returns a `Sink` and a `Stream` for outgoing and
incoming messages. The messages send into the `Sink` are forwarded down
to the network as well as send back into the `Stream` to ensure the node
processes its own messages.

So far, to send messages into the `Sink` back into the `Stream`, an
unbounded channel was used. This patch updates `round_communication` and
`OutgoingMessages` to use a bounded channel.

This is part of a greater effort to reduce the number of owners of
components within `finality-grandpa` and `network` as well as to reduce
the amount of unbounded channels. For details see d9837d7dd and
5f80929dc.

* client/finality-grandpa: Import futures03::future::ready at the top

* client/finality-grandpa: Make tests use compat of future 03

* client/finality-grandpa: Do not import ready into scope

Instead of importing futures03::future::ready into the scope, only
import futures::future03 into scope and call ready as furure03::ready.

substrate/client/finality-grandpa/src/environment.rs

@@ -22,7 +22,11 @@ use std::time::Duration;
 use log::{debug, warn, info};
 use parity_scale_codec::{Decode, Encode};
 use futures::prelude::*;
-use futures03::future::{FutureExt as _, TryFutureExt as _};
+use futures03::{
+	compat::{Compat, CompatSink},
+	future::{FutureExt as _, TryFutureExt as _},
+	stream::StreamExt as _,
+};
 use futures_timer::Delay;
 use parking_lot::RwLock;
 use sp_blockchain::{HeaderBackend, Error as ClientError};
@@ -608,6 +612,9 @@ where
 			has_voted,
 		);
 
+		let incoming = Compat::new(incoming.map(|item| Ok::<_, Error>(item)));
+		let outgoing = CompatSink::new(outgoing);
+
 		// schedule incoming messages from the network to be held until
 		// corresponding blocks are imported.
 		let incoming = Box::new(UntilVoteTargetImported::new(