fixing some warnings, organizing imports
splitting up ReservationEventBusImplTest

ReservationEventBusImplTest now only tests the downstream events because the dispatching of PortalBus events is done in the PortalEventDispatcherImpl (tests following)
adapting test cases for PortalEventDispatcher

adapting existing test cases from ReservationEventBus and adding further cases checking portal event store calls.
preventing concurrent modification of node sets

when calling removeAll on a set with another set as parameter, the second set is also modified. this can lead to a ConcurrentModificationException. Hence a new equal set is passed as parameter.