Fix ConcurrentModificationException in Broker

Change-Id: I645d65fdc414f2cf65703feec7a2ad06dc94960c
Reviewed-on: http://naan.mimuw.edu.pl:8001/108
Tested-by: Nebulostore Nebulostore <nebulostore@gmail.com>
Reviewed-by: Piotr Malicki <pmalicki126@gmail.com>
Tested-by: Piotr Malicki <pmalicki126@gmail.com>