PerformanceEvaluation: Decouple data size from client concurrency

PerfEval tool provides a --rows=R for specifying the number of records to work with and requires the user provide a value of N, used as the concurrency level. From what I can tell, every concurrent process will interact with R rows. In order to perform an apples-to-apples test, the user must re-calculate the value R for every new value of N.

Instead, I propose accepting a --size=S for the amount of data to interact with and let PerfEval divide that amongst the N clients on the user's behalf.