Comment 4 for bug 1352826

I changed the searching algorithm on my machine to reduce the complexity of _modify_rules() from O(n^2) to O(nlogn). Then this looping can complete in several seconds. Before proposing the new code, I hope to commit a unit test for _modify_rules() first in https://bugs.launchpad.net/neutron/+bug/1359072, so that we will not make _modify_rules() become wrong in the future.