Fix purge-chain and print DB info on startup (#5840)

* purge-chain accepts --db option

* print DB info on startup

* Small refactoring

* Added back &self

* Add DatabaseParams for PurgeChain, ImportParams and ExportBlocks

* Don't force default value

* Remove unused fields

* Update client/cli/src/commands/export_blocks_cmd.rs

* Fix stuff

Co-authored-by: Cecile Tonglet <cecile@parity.io>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/client/cli/src/commands/mod.rs

@@ -150,6 +150,12 @@ macro_rules! substrate_cli_subcommands {
 				}
 			}
 
+			fn database_params(&self) -> Option<&$crate::DatabaseParams> {
+				match self {
+					$($enum::$variant(cmd) => cmd.database_params()),*
+				}
+			}
+
 			fn base_path(&self) -> $crate::Result<::std::option::Option<::std::path::PathBuf>> {
 				match self {
 					$($enum::$variant(cmd) => cmd.base_path()),*