The method starts a task that uses sstables_loader load-and-stream
functionality to bring new sstables into the cluster. The existing
load-and-stream picks up sstables from upload/ directory, the newly
introduced task collects them from S3 bucket and given prefix (that
correspond to the path where backup API method put them).
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>