Switch wtperf from aggregating to sampling for latency measurements.
Stash the wtperf sample rate calculation in a local variable.
Merge branch 'develop' into wtperf-sample

Conflicts:
	bench/wtperf/wtperf.c
	bench/wtperf/wtperf_opt.i
Merge pull request #774 from wiredtiger/wtperf-sample

Switch wtperf from aggregating to sampling for latency measurements.