Code review comment for ~kissiel/plainbox:restartable-fixes

Yeah!
When I wrote the above patch, I checked usages of the restart_cmd_callback var. The only other use is when autorestart is configured, i.e. sa._restart_strategy is not None. And this is always set if client code called configure_application_restart (it's usually done, as you've mentioned from Launcher).

If you'd like I can add additional code asserting that's really the state (theoretically someone can still supply "None" as the callback from a new plainbox app, or by accident from a new patch to Launchers).