Stack flood rules are built based on the stack topo stated in the config
If the actual topo differs from the configured one, the flood rules
possibly won't work.
For example, Valve can't select a backup root if the primary one fails or
the selected root port hasn't run.
In order to support dynamic flood rules, I propose to (1) dynamically
update the stack topo,
and (2) support backup root DP and recalculate flood/unicast rules upon
This PR https://github.com/faucetsdn/faucet/pull/2055
is supposed to
The current flood algorithm may be used, but it will take into account the
actual topo when building rules.
Unicast rules may be updated upon topo change as well, or we could simply
delete them to re-learning hosts
(if we accept a temporarily unicast flooding).