Save status priority to charm state

[samuelallan72]

Priority for a state is lost across charm hook invocations in the following flow of events:

1. hook is invoked, StatusPool() is set up, statuses are dynamically added with manual priorities
2. statuses are set, and saved to charm state
3. hook is invoked again, StatusPool() is set up.
4. the status pool loads the saved statuses, with automatic sequencial prioritie
5. at this point, the custom priorities are lost, and will be ordered differently

We can fix this by saving the priority to the charm state, and restoring the priority when restoring.