* secret_store: remove kvdb_rocksdb dependency
* cli: init db mod for open dispatch
* cli: move db, client_db, restoration_db, secretstore_db to a separate mod
* migration: rename to migration-rocksdb and remove ethcore-migrations
* ethcore: re-move kvdb-rocksdb dep to test
* mark test_helpers as test only and fix migration mod naming
* Move restoration_db_handler to test_helpers_internal
* Fix missing preambles in test_helpers_internal and rocksdb/helpers
* Move test crates downward
* Fix missing docs
* cli, db::open_db: move each argument to a separate line
* Use featuregate instead of dead code for `open_secretstore_db`
* Move pathbuf import to open_secretstore_db
Because it's only used there behind a feature gate
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