Support non-default datadir with rebuild-server

Allow non-default server datadir. The async-rebuild-server and async-restore-from-backup commands should ask the server its datadir. Alternatively, allow specifying it manually. Also, Mariabackup memory use and thread count should be configurable.

Original description:
Is there anyway to use MariaDB monitor's rebuild-server option but have the data go to a different directory than /var/lib/mysql? Can it read the datadir and binlog directories from the target server before it stops MariaDB-server on the target machine. Maybe also clear out the temp directory.