Experimental. Not automatically executed.

Restores given backup, configures service as a slave to given master,
starts replication.

Like for "normal" backup restoration, any running mariadb is killed,
started with backup-restoration-friendly parameters, and killed again once
restored.

As there is AFAIK no way to ask supervisor to start even one of our own
services, this script unfortunately leaves the partition in a
not-immediately useful state.