added WorkloadStreams class <-- will be return type of Workload
rewrite WorkloadRunner to use WorkloadStream instead of Iterators directly
remove all iterator splitting from WorkloadRunner
remove usage of windowed executor (to be reintroduced later, redesigned to be more efficient using lesson learned, after benchmarking without it)
rewrite PreciseIndividualAsyncOperationStreamExecutorServiceThread to use new WorkloadStream abstraction