Slf null performance enhancements

The commit comments tell almost all of the story:

* Switch timing from erlang:now/0 -> os:timestamp/0
* Change process priority of basho_bench_stats proc to high AND Create more regular reporting intervals in our CSV files
  * To avoid problems where {report_interval,1} is used but the CSV files have entries every 41 seconds
* 	 Reduce spamminess of seq_gen_state_dir check
* Add config option: disable_sequential_int_progress_report
  * The additional work created by lager event logging can be significant enough to want disabling