[Gridmix] Improve cumulative CPU usage emulation for short running tasks

CPU emulation in Gridmix fails to meet the expected target if the map has no data to sort/spill/merge. There are 2 major reasons for this:
1. The map task end immediately ends soon after the map task. The map progress is 67% while the map phase ends.
2. Currently, the sort (comparator) doesnt emulate CPU. If the map is short lived, the CPU emulation thread (spawned from the map task in cleanup) doesn't get a chance to emulate.