Unify and limit rocksdb dependency places (#8371)

* 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