* Remove usage of wsrep_provider variable in galera_ist_restart_joiner
* Rename galera_load_provider.inc and galera_unload_provider.inc to
  galera_stop_replication.inc and galera_start_replication.inc. Their
  original names were no longer reflecting what these include files do.

followup for ce3a2a68Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>