vecsum: set normal chunk = 8 MB, don't stopwatch on failure
vecsum: set normal chunk = 8 MB, don't stopwatch on failure

Signed-off-by: Colin Patrick McCabe <3142ce8a0bdba4a38073365893e6b74f72d9c512@cloudera.com>

umbrant/caching_benchmarking,umbrant/caching_benchmarking,umbrant/caching_benchmarking,umbrant/caching_benchmarking