Use type alias for transactions (paritytech#6431)

Very tiny change that helps with debugging of transactions propagation
by referring to the same type alias not only at receiving side, but also
on the sending size for symmetry

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

@@ -480,7 +480,7 @@ where
 				continue
 			}
 
-			let (hashes, to_send): (Vec<_>, Vec<_>) = transactions
+			let (hashes, to_send): (Vec<_>, Transactions<_>) = transactions
 				.iter()
 				.filter(|(hash, _)| peer.known_transactions.insert(hash.clone()))
 				.cloned()