Comment 9 for bug 1253993

Apart from the neturon server side RPCs, we observed in our scale tests that IPtables Manager _apply method, takes significant amount of time, as the number of devices for which the firewall refresh is needed increases. Especially the "_modify_rules" method .
Using list comprehensions in this method is adding to the delay. I have modified that particular code and saw good improvements.
Will try posting the changes for review at the earliest.