Mv level work2

This branch provides further tuning to the mv-level-work1 branch.  The tuning is focused upon reducing when write throttle penalties apply and the performance between levels 2 and 3.  The level 2 to 3 transition is when the code shifts from overlapped .sst files to sorted.  The change slightly increases the write amplification (bad) but reduces latencies, both overall and worst case (good).