Re-organize QueryEvalStats class and collection of these stats

[boeker]

Splits the class QueryEvalStats into QuerySetupStats (not thread safe, written to in NearestNeighborBlueprint) and QueryEvalStats (thread safe, written to in ExactNearestNeighborIterator). The ANN stats in QueryEvalStats were atomic before, but I do not see a reason for this as we only write to these in the query setup and not from the search iterators.

The class QuerySetupStats is then wired into the perform_ann_searches method to avoid the awkward flushing in the NearestNeighborBlueprint (the QueryEvalStats was and is not installed when performing the ANN searches).

[boeker]

Maybe I should call the QuerySetupStats class AnnStats or something like that instead since it is so specific. 🤔