- Automatically re-index imported retained messages during restoring a data backup file. Previously, it was needed to manually trigger re-indexing with `emqx ctl retainer reindex start` CLI
  after the data backup file is imported.

- Allow exporting retained messages to a backup file if the configured storage_type (`retainer.backend.storage_type`) is `ram`. Previously, retained messages could be exported only if `disc` storage_type was configured.