Fixed kill option

At a kill_cycle the threads noow just delete al versions of the
current file rather than every file. Thsi stops the whole workign set
getting trashed so often that it never gets properly loaded.

Threads cycle through files at each read or write. This ensures that
when a kill occurs the deletes are spread out more widely, ensuring
that a reasonable proportion of the data set gets trashed when we hit
a kill cycle (threads tend to hit the cycle at roughly the same time).