Starts no children if yokozuna is not enabled (start_link(false)).
Starts two sub-supervisors if yokozuna is enabled: one supervisor
for the JVM solr manager, and another supervisor for the rest of
the long-lived yokozuna processes. This top-level supervisor's
restart strategy is thus to allow zero restarts of its
sub-supervisors. If those sub-supervisors exit, something is really
wrong, and yokozuna should shut down.
