[1174841] UnsupportedOperationException on mergeInventoryReport()

Avoid the use of Iterator.remove() because Resources coming from the
Agent may be using a customized impl for Resource.childResources (like
CopyOnWriteArraySet).  The solution "lazily protects" because the
problem scenario is rare (restype reported by agent is not
present on the server) and we don't want to do any unnecessary work (like
changing the Set impl in advance).