improve startup and runtime performance by

1) moving common startup procedures out of hotplug script when called
from mwan3 start
2) reducing calls to iptables to check status of rules
3) consolidating iptables updates and updating with iptables-restore
4) do not wait for kill if nothing was killed
5) running interface hotplug scripts in parallel
6) eliminate operations in hotplug script that check status on every
single interface unnecessarily
7) consolidate how mwan3track makes hotplug calls
8) do not restart mwan3track on connected events

This is a significant refactor, but should not result in any breaking
changes or require users to update their configurations.

version bump to 2.9.0

Signed-off-by: Aaron Goodman <aaronjg@stanford.edu>