- Fix to avoid mysqltest client getting killed abruptly during
  mysql_shutdown(). When Galera replication is shutdown, wait for
  THDs with `thd->stmt_da()->is_eof()` to disconnect (these are about
  to disconnect anyway).
- Extract duplicate code from `wsrep_stop_replication()` and
  `wsrep_shutdown_replication()` in a new function.
- No need to use a custom `shutdown_mysqld.inc` in galera
  suite. Delete it, so that the one in `mysql-test/include/` is used.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>