rpc-v2: Limit transactionBroadcast calls to 16 (#3772)

This PR limits the number of active calls to the transactionBroadcast
APIs to 16.

cc @paritytech/subxt-team 

Closes: https://github.com/paritytech/polkadot-sdk/issues/3081

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: James Wilson <james@jsdw.me>

substrate/client/service/src/builder.rs

@@ -644,10 +644,13 @@ where
 		(chain, state, child_state)
 	};
 
+	const MAX_TRANSACTION_PER_CONNECTION: usize = 16;
+
 	let transaction_broadcast_rpc_v2 = sc_rpc_spec_v2::transaction::TransactionBroadcast::new(
 		client.clone(),
 		transaction_pool.clone(),
 		task_executor.clone(),
+		MAX_TRANSACTION_PER_CONNECTION,
 	)
 	.into_rpc();