Store OffersMessages for later sending

Upcoming commits will add utilities for sending an InvoiceRequest for an
Offer and an Invoice for a Refund. These messages need to be enqueued so
that they can be released in ChannelManager's implementation of
OffersMessageHandler to OnionMessenger for sending.

These messages do not need to be serialized as they must be resent upon
restart.

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -59,6 +59,7 @@ use crate::ln::wire::Encode;
 use crate::offers::offer::{DerivedMetadata, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::RefundBuilder;
+use crate::onion_message::{OffersMessage, PendingOnionMessage};
 use crate::sign::{EntropySource, KeysManager, NodeSigner, Recipient, SignerProvider, WriteableEcdsaChannelSigner};
 use crate::util::config::{UserConfig, ChannelConfig, ChannelConfigUpdate};
 use crate::util::wakers::{Future, Notifier};
@@ -964,33 +965,35 @@ where
 //
 // Lock order tree:
 //
+// `pending_offers_messages`
+//
 // `total_consistency_lock`
 //  |
 //  |__`forward_htlcs`
 //  |   |
 //  |   |__`pending_intercepted_htlcs`
 //  |
 //  |__`per_peer_state`
-//  |   |
-//  |   |__`pending_inbound_payments`
-//  |       |
-//  |       |__`claimable_payments`
-//  |       |
-//  |       |__`pending_outbound_payments` // This field's struct contains a map of pending outbounds
-//  |           |
-//  |           |__`peer_state`
-//  |               |
-//  |               |__`id_to_peer`
-//  |               |
-//  |               |__`short_to_chan_info`
-//  |               |
-//  |               |__`outbound_scid_aliases`
-//  |               |
-//  |               |__`best_block`
-//  |               |
-//  |               |__`pending_events`
-//  |                   |
-//  |                   |__`pending_background_events`
+//      |
+//      |__`pending_inbound_payments`
+//          |
+//          |__`claimable_payments`
+//          |
+//          |__`pending_outbound_payments` // This field's struct contains a map of pending outbounds
+//              |
+//              |__`peer_state`
+//                  |
+//                  |__`id_to_peer`
+//                  |
+//                  |__`short_to_chan_info`
+//                  |
+//                  |__`outbound_scid_aliases`
+//                  |
+//                  |__`best_block`
+//                  |
+//                  |__`pending_events`
+//                      |
+//                      |__`pending_background_events`
 //
 pub struct ChannelManager<M: Deref, T: Deref, ES: Deref, NS: Deref, SP: Deref, F: Deref, R: Deref, L: Deref>
 where
@@ -1196,6 +1199,8 @@ where
 	event_persist_notifier: Notifier,
 	needs_persist_flag: AtomicBool,
 
+	pending_offers_messages: Mutex<Vec<PendingOnionMessage<OffersMessage>>>,
+
 	entropy_source: ES,
 	node_signer: NS,
 	signer_provider: SP,
@@ -2266,6 +2271,8 @@ where
 			event_persist_notifier: Notifier::new(),
 			needs_persist_flag: AtomicBool::new(false),
 
+			pending_offers_messages: Mutex::new(Vec::new()),
+
 			entropy_source,
 			node_signer,
 			signer_provider,
@@ -9852,6 +9859,8 @@ where
 			event_persist_notifier: Notifier::new(),
 			needs_persist_flag: AtomicBool::new(false),
 
+			pending_offers_messages: Mutex::new(Vec::new()),
+
 			entropy_source: args.entropy_source,
 			node_signer: args.node_signer,
 			signer_provider: args.signer_provider,