Store AsyncPaymentsMessages for later sending

Upcoming commits will support sending and receiving held_htlc_available and
release_held_htlc messages. These messages need to be enqueued so that they can
be released in ChannelManager's implementation of AsyncPaymentsMessageHandler
to OnionMessenger for sending.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -1932,6 +1932,8 @@ where
 //
 // `pending_offers_messages`
 //
+// `pending_async_payments_messages`
+//
 // `total_consistency_lock`
 //  |
 //  |__`forward_htlcs`
@@ -2184,6 +2186,7 @@ where
 	needs_persist_flag: AtomicBool,
 
 	pending_offers_messages: Mutex<Vec<PendingOnionMessage<OffersMessage>>>,
+	pending_async_payments_messages: Mutex<Vec<PendingOnionMessage<AsyncPaymentsMessage>>>,
 
 	/// Tracks the message events that are to be broadcasted when we are connected to some peer.
 	pending_broadcast_messages: Mutex<Vec<MessageSendEvent>>,
@@ -2978,6 +2981,7 @@ where
 			funding_batch_states: Mutex::new(BTreeMap::new()),
 
 			pending_offers_messages: Mutex::new(Vec::new()),
+			pending_async_payments_messages: Mutex::new(Vec::new()),
 			pending_broadcast_messages: Mutex::new(Vec::new()),
 
 			last_days_feerates: Mutex::new(VecDeque::new()),
@@ -12421,6 +12425,7 @@ where
 			funding_batch_states: Mutex::new(BTreeMap::new()),
 
 			pending_offers_messages: Mutex::new(Vec::new()),
+			pending_async_payments_messages: Mutex::new(Vec::new()),
 
 			pending_broadcast_messages: Mutex::new(Vec::new()),