Switch wtperf from aggregating to sampling for latency measurements.
Use the same timestamp format in wtperf output as in statistics_log
output, that way it's easy to match the graphs against each other.

Fix a couple of trivial typos in statistics strings.
Fix a leak of key/value buffers in wtperf.  While in the area, rename "data_buf" to "value_buf" to match the WT API.
s/data/value/ in a few more places.
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.
lint, mark the /* FALLTHROUGH */ case.
Info 768: global struct member 'aggregated' (line 124, file
wtperf.h) not referenced
Remove unnecessary __wt_epoch() call, the indentation makes it look
like an accident.
Remove another unnecessary __wt_epoch call; now that we're no longer
switching back-and-forth between time structures, rename them to
"start" and "stop" and get rid of the pointer versions.
Change the default sample rate in wtperf to 50 -- sampling every operation introduced some big regressions on the EC2 box.
Cleanup in wtperf including:

* Stop using asserts, handle errors correctly
* Switch all gettime calls to use wt_epoch
* Only track latencies if sample_interval is configured
Fix compiler warning in wtperf:

../../../bench/wtperf/wtperf.c:789:27: error: conversion to 'long long
unsigned int' from 'long int' may change the sign of the result
[-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c:789:13: error: conversion to 'long long
unsigned int' from 'time_t' may change the sign of the result
[-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c:790:30: error: conversion to 'long long
unsigned int' from 'long int' may change the sign of the result
[-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c:790:15: error: conversion to 'long long
unsigned int' from 'time_t' may change the sign of the result
[-Werror=sign-conversion]
The checkpoint thread gets the current time-of-day, but does nothing
with it -- remove the retrieval.
Remove unused ms calculation.
lint: typo, parenthesis in the wrong place