Implements CollectionSetVisitor to implement thresholding.
Works by simply recording all the attributes that come in via visitAttribute
into an internal data structure, per resource, and then on "completeResource", does
threshold checking against that in memory structure.
Suggested usage is one per CollectableService; this object holds the current state of thresholds
for this interface/service combination
(so perhaps needs a better name than ThresholdingVisitor)
Assumes and requires that the any visitation start at CollectionSet level, so that the collection timestamp can