Pool: parallel ready and runtime changes (#922)

* Revert "Revert runtime changes."

This reverts commit 01a7d1aa83c2918dd63b7dc54eb688d544cfc649.

* Parallel queue reads.

* Avoid recursion in best iterator.

substrate/core/test-runtime/src/system.rs

@@ -131,12 +131,12 @@ pub fn validate_transaction(utx: Extrinsic) -> TransactionValidity {
 		p
 	};
 
-	TransactionValidity::Valid(
-		/* priority: */tx.amount,
+	TransactionValidity::Valid {
+		priority: tx.amount,
 		requires,
 		provides,
-		/* longevity: */64
-	)
+		longevity: 64
+	}
 }