Implement only sending one notification at a time as per RFC 56 (#2813)

cc https://github.com/paritytech/polkadot-sdk/issues/2812
cc https://github.com/polkadot-fellows/RFCs/pull/56

Since this is a one line of code change, and for the sake of this not
taking six months to be done, I've opted to open a PR myself.

---------

Co-authored-by: Aaro Altonen <48052676+altonen@users.noreply.github.com>

prdoc/pr_2813.prdoc

+title: "Implement only sending one notification at a time as per RFC 56"
+
+doc:
+  - audience: Node Dev
+    description: |
+      Transactions are now gossiped one at a time instead of as batches, as per RFC 56. This
+      allows decoding notifications without knowing how to decode individual transactions, and
+      allows for a more fine grained backpressure.
+
+crates:
+  - name: "sc-network-transactions"

substrate/client/network/transactions/src/lib.rs

@@ -475,7 +475,20 @@ where
 					propagated_to.entry(hash).or_default().push(who.to_base58());
 				}
 				trace!(target: "sync", "Sending {} transactions to {}", to_send.len(), who);
-				let _ = self.notification_service.send_sync_notification(who, to_send.encode());
+				// Historically, the format of a notification of the transactions protocol
+				// consisted in a (SCALE-encoded) `Vec<Transaction>`.
+				// After RFC 56, the format was modified in a backwards-compatible way to be
+				// a (SCALE-encoded) tuple `(Compact(1), Transaction)`, which is the same encoding
+				// as a `Vec` of length one. This is no coincidence, as the change was
+				// intentionally done in a backwards-compatible way.
+				// In other words, the `Vec` that is sent below **must** always have only a single
+				// element in it.
+				// See <https://github.com/polkadot-fellows/RFCs/blob/main/text/0056-one-transaction-per-notification.md>
+				for to_send in to_send {
+					let _ = self
+						.notification_service
+						.send_sync_notification(who, vec![to_send].encode());
+				}
 			}
 		}