mapbench: start timer thread along with workers
mapbench: start timer thread along with workers

Otherwise at 80 cores the timer thread would start counting nearly
0.5 seconds before all of the threads got going, making it hard to
tell whether the benchmark was really scaling by comparing page
touch counts (although the reported cycles value was accurate).

bowlofstew/sv6,aclements/sv6,aclements/sv6,aclements/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6