o/snapstate: in hybrid systems rerefresh before reboot if possible

[miguelpires]

In hybrid systems where we do refreshes of apps before the reboot and essential snaps after, check if a re-refresh is necessary after the apps are done instead of waiting for all (and therefore a reboot). This is possible because currently essential snaps don't use epochs and don't require re-refreshes. If an app has a dependency that requires a reboot (like the model base), then the check-rerefresh task needs to wait for that refresh to complete. Since, if we're waiting for the reboot, there isn't much to gain in resolving the app's dependencies to only wait for those, the check-rerefresh preserves the old behavior in that case to keep things simple.