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/params/database_params.rs

@@ -0,0 +1,47 @@
+// Copyright 2018-2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::arg_enums::Database;
+use structopt::StructOpt;
+
+/// Parameters for block import.
+#[derive(Debug, StructOpt, Clone)]
+pub struct DatabaseParams {
+	/// Select database backend to use.
+	#[structopt(
+		long,
+		alias = "db",
+		value_name = "DB",
+		case_insensitive = true,
+	)]
+	pub database: Option<Database>,
+
+	/// Limit the memory the database cache can use.
+	#[structopt(long = "db-cache", value_name = "MiB")]
+	pub database_cache_size: Option<usize>,
+}
+
+impl DatabaseParams {
+	/// Limit the memory the database cache can use.
+	pub fn database(&self) -> Option<Database> {
+		self.database
+	}
+
+	/// Limit the memory the database cache can use.
+	pub fn database_cache_size(&self) -> Option<usize> {
+		self.database_cache_size
+	}
+}