* 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>