Core attributes should be guarded

Evaluating some work for UpdateAttributes, specifically the ability to delete attributes, led to being able to delete UUID (https://github.com/apache/nifi/pull/116)

As a result, the framework was nonplussed at the missing core attribute, uuid:

12:53:35 ESTERROR6d65c083-eb22-4ae7-b542-de808b1402d0
UpdateAttribute[id=6d65c083-eb22-4ae7-b542-de808b1402d0] UpdateAttribute[id=6d65c083-eb22-4ae7-b542-de808b1402d0] failed to process session due to java.lang.IllegalStateException: Cannot create Provenance Event Record because FlowFile UUID is not set: java.lang.IllegalStateException: Cannot create Provenance Event Record because FlowFile UUID is not set

Not sure if this applies to other core attributes, but at minimum, it seems like the UUID needs to have some framework level guarding against it from being inappropriately modified.