refactored sub-result set extraction into SliceQuery and added limit check.
Added StoreFeatures option supportsMultiQuery so that backends can indicate whether they can efficiently execute (i.e. bundle) multiQueries.
Add graph level configuration option parallel-backend-ops, which - when enabled - will create a thread pool that a backend transaction uses to parallelize multiQueries for backends that don't support them natively.
Added check in testGetSlices() for whether the store supports multiQuery