rpc server: make possible to disable/enable batch requests (#3364)

The rationale behind this, is that it may be useful for some users
actually disable RPC batch requests or limit them by length instead of
the total size bytes of the batch.

This PR adds two new CLI options:

```
--rpc-disable-batch-requests - disable batch requests on the server
--rpc-max-batch-request-len <LEN> - limit batches to LEN on the server.
```

substrate/client/cli/src/config.rs

@@ -28,7 +28,8 @@ use sc_service::{
 	config::{
 		BasePath, Configuration, DatabaseSource, KeystoreConfig, NetworkConfiguration,
 		NodeKeyConfig, OffchainWorkerConfig, OutputFormat, PrometheusConfig, PruningMode, Role,
-		RpcMethods, TelemetryEndpoints, TransactionPoolOptions, WasmExecutionMethod,
+		RpcBatchRequestConfig, RpcMethods, TelemetryEndpoints, TransactionPoolOptions,
+		WasmExecutionMethod,
 	},
 	BlocksPruning, ChainSpec, TracingReceiver,
 };
@@ -338,7 +339,12 @@ pub trait CliConfiguration<DCV: DefaultConfigurationValues = ()>: Sized {
 		Ok(RPC_DEFAULT_MESSAGE_CAPACITY_PER_CONN)
 	}
 
-	/// Rate limit calls per minute.
+	/// RPC server batch request configuration.
+	fn rpc_batch_config(&self) -> Result<RpcBatchRequestConfig> {
+		Ok(RpcBatchRequestConfig::Unlimited)
+	}
+
+	/// RPC rate limit configuration.
 	fn rpc_rate_limit(&self) -> Result<Option<NonZeroU32>> {
 		Ok(None)
 	}
@@ -515,6 +521,7 @@ pub trait CliConfiguration<DCV: DefaultConfigurationValues = ()>: Sized {
 			rpc_max_subs_per_conn: self.rpc_max_subscriptions_per_connection()?,
 			rpc_port: DCV::rpc_listen_port(),
 			rpc_message_buffer_capacity: self.rpc_buffer_capacity_per_connection()?,
+			rpc_batch_config: self.rpc_batch_config()?,
 			rpc_rate_limit: self.rpc_rate_limit()?,
 			prometheus_config: self
 				.prometheus_config(DCV::prometheus_listen_port(), &chain_spec)?,