Refactor RelayList

Refactor 1: It should probably have a dictionary of relays instead of a list, so that it is not needed to convert the list to dictionary in resultdump. It should be then named Relays or RelayDict In https://github.com/torproject/sbws/issues/219 it was proposed to change the list to a set, but that's only used to exclude authorities in the prioritizer, which should also be a method in RelayList.