Run "accounting_run_housekeeping" with a periodic event.

Here's an implementation sketch:

• Make accounting_add_bytes() check whether limits are exceeded. Refactor the code as needed to make this check fast.
• If a limit is exceeded, schedule consider_hibernation().
• Also schedule consider_hibernation() again based on "shutdown_time", "hibernate_end_time", and "interval_wakeup_time".
• Pull "accounting_run_housekeeping" apart:
  • Other logic to set interval_end_time if it isn't set but accounting is enabled.
  • A periodic event, only scheduled when network is on and accounting is enabled, to record bandwidth usage. Tie this into the or_state_save event.