Exchange objectives can no longer be assigned to AIs and double agents.

-Includes a much simpler solution to preventing latejoining exchangers
-Exchange objectives also support additional objectives now (ie if config.traitor_objectives_amount > 1)
-Exhange and hijacking objectives count towards the number of objectives (ie if config.traitor_objectives_amount > 1)
Update traitor.dm
Merge pull request #4102 from Ikarrus/aiscantexchangesilly

Exchange objectives can no longer be assigned to AIs and double agents.