CLI improvements & fixes (#4812)

These are a few changes I missed during the refactoring.

1. Initialization issue and boilerplate

    Most importantly: part of the `Configuration` initialization was done in `sc_cli::init`. This means the user can not benefit from this initialization boilerplate if they have multiple `Configuration` since `sc_cli::init` can only be called once.

2. Boilerplate for `VersionInfo` and `Configuration`

    I'm also answering to the critic of @bkchr on the initialization using version: https://github.com/paritytech/substrate/pull/4692/files/bea809d4c14a2ede953227ac885e3b3f9771c548#r372047238 This will allow initializing a `Configuration` and provide the version by default.

3. Loading the `chain_spec` explicitly

    In the past it was done automatically but in some cases we want to delay this. I moved the code to `Configuration.load_spec()` so it can be called later on. `chain_spec` can also be written directly to the `Configuration` without using this `load_spec` helper.

4. [deleted]

5. Fixing issue that prevents the user to override the port

    In the refactoring I introduced a bug by mistake that could potentially prevent the CLI user to override the ports if defaults where provided for these ports (only on cumulus).

6. Change task_executor from Box to Arc

    This is useful for cumulus where we have 2 nodes with 2 separate Configuration that need to spawn tasks to the same runtime.

7. Renamed TasksExecutorRequired to TaskExecutor

    For consistency.

This is related to https://github.com/paritytech/cumulus/issues/24

This is the continuation (and hopefully the end of) #4692

substrate/bin/node/cli/src/command.rs

@@ -28,8 +28,7 @@ where
 	let args: Vec<_> = args.collect();
 	let opt = sc_cli::from_iter::<Cli, _>(args.clone(), &version);
 
-	let mut config = sc_service::Configuration::default();
-	config.impl_name = "substrate-node";
+	let mut config = sc_service::Configuration::new(&version);
 
 	match opt.subcommand {
 		None => sc_cli::run(
@@ -41,8 +40,8 @@ where
 			&version,
 		),
 		Some(Subcommand::Factory(cli_args)) => {
-			sc_cli::init(&mut config, load_spec, &cli_args.shared_params, &version)?;
-
+			sc_cli::init(&cli_args.shared_params, &version)?;
+			sc_cli::load_spec(&mut config, &cli_args.shared_params, load_spec)?;
 			sc_cli::fill_import_params(
 				&mut config,
 				&cli_args.import_params,