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/client/service/src/builder.rs

@@ -196,7 +196,7 @@ fn new_full_parts<TBl, TRtApi, TExecDisp, TGen, TCSExt>(
 			state_cache_child_ratio:
 			config.state_cache_child_ratio.map(|v| (v, 100)),
 			pruning: config.pruning.clone(),
-			source: match &config.database {
+			source: match config.expect_database() {
 				DatabaseConfig::Path { path, cache_size } =>
 					sc_client_db::DatabaseSettingsSrc::Path {
 						path: path.clone(),
@@ -307,7 +307,7 @@ where TGen: RuntimeGenesis, TCSExt: Extension {
 				state_cache_child_ratio:
 					config.state_cache_child_ratio.map(|v| (v, 100)),
 				pruning: config.pruning.clone(),
-				source: match &config.database {
+				source: match config.expect_database() {
 					DatabaseConfig::Path { path, cache_size } =>
 						sc_client_db::DatabaseSettingsSrc::Path {
 							path: path.clone(),
@@ -1187,7 +1187,7 @@ ServiceBuilder<
 			task_executor: if let Some(exec) = config.task_executor {
 				exec
 			} else {
-				return Err(Error::TasksExecutorRequired);
+				return Err(Error::TaskExecutorRequired);
 			},
 			rpc_handlers,
 			_rpc: rpc,