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.
Fix a clang signed / unsigned conversion warning.
Don't read past the end of the chunk array: wait until we know there are enough chunks before reading.
Fix a tiny leak in Bloom filter creation, found by valgrind.  We could allocate space for the config string twice.