When removing PeerState check for in-flight mon updates deeply

When deciding if we should remove a `PeerState` entry we want to
ensure we don't remove if there are pending updates in
`in_flight_monitor_updates`. Previously this was done with a simple
`in_flight_monitor_updates.is_empty()`, however this can prevent
removal of `PeerState` entries if a channel had an update at some
point (leaving an entry in the map) but the update was ultimately
completed.

Instead, we need to iterate over the entries in
`in_flight_monitor_updates` and decline to remove `PeerState`s only
if there is an entry for a pending update still in-flight.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -1332,6 +1332,11 @@ impl <SP: Deref> PeerState<SP> where SP::Target: SignerProvider {
 		if require_disconnected && self.is_connected {
 			return false
 		}
+		for (_, updates) in self.in_flight_monitor_updates.iter() {
+			if !updates.is_empty() {
+				return false;
+			}
+		}
 		!self.channel_by_id.iter().any(|(_, phase)|
 			match phase {
 				ChannelPhase::Funded(_) | ChannelPhase::UnfundedOutboundV1(_) => true,
@@ -1341,7 +1346,6 @@ impl <SP: Deref> PeerState<SP> where SP::Target: SignerProvider {
 			}
 		)
 			&& self.monitor_update_blocked_actions.is_empty()
-			&& self.in_flight_monitor_updates.is_empty()
 			&& self.closed_channel_monitor_update_ids.is_empty()
 	}