Fixed bug that caused stalls in ParallelRun.h
Fixed bug that caused stalls in ParallelRun.h

Didn't properly keep track of the number of outstanding tasks
(empty tasks were counted as outstanding), which caused progress
to stall indefinitely if the number of empty tasks exceeded
maxOutstanding.

utah-scs/RAMCloud,anirajk/RAMCloud,utah-scs/RAMCloud,anirajk/RAMCloud,utah-scs/RAMCloud,SMatsushi/RAMCloud,anirajk/RAMCloud,SMatsushi/RAMCloud,anirajk/RAMCloud,SMatsushi/RAMCloud,jblomer/ramcloud,anirajk/RAMCloud,utah-scs/RAMCloud,jblomer/ramcloud,jblomer/ramcloud,jblomer/ramcloud,jblomer/ramcloud,SMatsushi/RAMCloud,anirajk/RAMCloud,utah-scs/RAMCloud,SMatsushi/RAMCloud,utah-scs/RAMCloud,jblomer/ramcloud,SMatsushi/RAMCloud