What is being done to stop assassins from dominating mid lane?
The bounty changes have pushed them off the deep end, not that they weren't already completely dominating mid lane. It's no longer rewarding to beat an assassin in lane.
If they pick up shutdown gold, either by suiciding into you for a kill or roaming to another lane, they suddenly go from being 0-4-0 and behind to being capable of one shotting you if you leave your turret.