This applies only to an empty cluster and makes the cluster transition
from the initial state RECOVERING to BACKINGUP without passing through
state RUNNING:

(empty) -> RECOVERING -> VERIFYING -> STARTING_BACKUP -> BACKINGUP

This is the only way to reach BACKINGUP without having to pass through
RUNNING first and manually transitioning to STARTING_BACKUP. It is the
only way to reach state BACKINGUP fully automatically.

See merge request !25