Change drcachesim default to core_serial for a much improved schedule

[derekbruening]

Today the drcachesim default is still the old static round-robin assignment of threads to cores. We should change the default to be core_serial to use the scheduler. There are complexities preventing us from removing the old static schedule, though: if you want to run another tool at the same time that doesn't want core-sharded, it seems like we should still support that.