Code review comment for ~blake-rouse/maas:improved-dns-reload

there are no concurrent reloads, but there is no throttling, e.g. if 2 events within the same second arrive, maas will run 'rndc reload' twice, while it could just run it once at the end of processing of the second event (the last that finishes it task), that's what it's suggested in the bug description:

""" - Throttle consecutive updates. (For example, when the first DNS change occurs, schedule a task for a few seconds in the future which will update DNS, and don't ever schedule more additional updates while that task is pending.)"""