Switch statistics to rdtsc from epoch calls

In WT-3133 a new function wt_rdtsc was added to get a clock representation that was very fast. However in considering using it to replace more expensive calls to wt_epoch for timing calculations reveals that this is not a simple process. We should add a function and backend support code to convert rdtsc of clock cycles into nanoseconds. We should also compare and consider whether rdtscp is a better instruction.