Limit transaction pool size (#1676)

* Avoid excessive hashing. Store extrinsic len.

* Implement pool limits.

* Fix issues.

* Make sure we return error in case it doesn't make into the pool.

* Pass parameters from CLI.

* Remove redundant todo.

* Fix tests.

substrate/core/transaction-pool/src/tests.rs

@@ -68,8 +68,9 @@ impl txpool::ChainApi for TestApi {
 		})
 	}
 
-	fn hash(&self, ex: &txpool::ExtrinsicFor<Self>) -> Self::Hash {
-		BlakeTwo256::hash(&ex.encode())
+	fn hash_and_length(&self, ex: &txpool::ExtrinsicFor<Self>) -> (Self::Hash, usize) {
+		let encoded = ex.encode();
+		(BlakeTwo256::hash(&encoded), encoded.len())
 	}
 
 }