Calculate execution time in benchmarks using System.nanoTime() instead of System.currentTimeMillis()

[asqasq]

CrailBenchmark and HdfsIOBenchmark should calculate the execution time on nanoseconds instead of milliseconds (see PR 35).

Example:
[...]
long end = System.currentTimeMillis();
double executionTime = ((double) (end - start));
double latency = executionTime*1000.0 / ((double) batch);
[...]