wsrep_sst_method for BACKUP SERVER

To benefit from the faster streaming BACKUP SERVER WITH (MDEV-35362), we need a wsrep_sst_method based on it and tar, instead of mariadb-backup and mbstream. For some reason, the mariadb-backup compatible wrapper that is being introduced in MDEV-39061 does not work out of the box.

As a first cut, we could use a single-threaded, single-stream backup.

While working on this, it would be useful to check what galera.galera_sst_mariabackup is spending its time on. The test takes more than 60 seconds of wall-clock time, and there is not much CPU activity going on.