vswitchd: Reduce number of calls to reconfigure() during mgmt updates

vswitchd: Reduce number of calls to reconfigure() during mgmt updates

When we receive an OpenFlow management protocol Config Update, we
immediately force the switch to reconfigure itself.  This is
functionally correct, but it can cause long delays before return control
back to the switch.  We now keep track of whether there were any changes
and then only force a reconfigure once per management run.